Biomarkers

8-OHdG (8-hydroxy-2'-deoxyguanosine) is an oxidatively modified nucleoside formed when reactive oxygen species (ROS) attack the C8 position of guanine in DNA; it is among the most abundant and best-characterized oxidative DNA lesions in nuclear and mitochondrial DNA. When OGG1 (8-oxoguanine DNA glycosylase-1) excises the damaged base, the nucleoside is released into circulation and excreted in urine, making urinary 8-OHdG — normalized to creatinine — a non-invasive readout of oxidative DNA damage and repair activity. Levels rise with age, are elevated in smokers, obese individuals, and those exposed to ionizing radiation or pollutants, and are consistently higher in patients with cancer, atherosclerosis, type 2 diabetes, and neurodegenerative disease than in healthy controls (Wu et al., 2004). These associations are observational; 8-OHdG reflects oxidative burden but has not been established as a causal driver of disease in humans, and antioxidant interventions that lower 8-OHdG have not demonstrated proven clinical benefit. Measurement is most reliable by LC-MS/MS or HPLC-ECD; commercial ELISA kits show cross-reactivity artifacts that can substantially inflate values relative to chromatographic methods, with discrepancies varying across kits and sample matrices (Korkmaz et al., 2018; Hu et al., 2004). In prospective cohort research, urinary 8-OHdG correlates only weakly with chronological age and does not reliably distinguish accelerated from normal biological aging, limiting its standalone utility as a longevity biomarker (Mukli et al., 2022).

Albumin is the most abundant plasma protein, synthesized exclusively by the liver, and maintains colloid osmotic pressure while transporting hormones, fatty acids, calcium, bilirubin, and many drugs. Serum levels reflect hepatic synthetic capacity, nutritional status, inflammation, and renal or gastrointestinal losses. Lower albumin is a robust marker of biological aging and is consistently associated with sarcopenia, frailty, longer hospital stays, and higher all-cause mortality, which is why it features in many composite aging indices.

The albumin/globulin ratio (A/G ratio) is calculated from total protein and albumin measurements as albumin divided by (total protein minus albumin), with the globulin fraction encompassing immunoglobulins, complement proteins, acute-phase reactants, and transport proteins. In healthy adults the A/G ratio typically ranges from ~1.1 to 2.5; a lower ratio can result from reduced albumin (malnutrition, liver disease, nephrotic syndrome) or from elevated globulins (chronic inflammation, multiple myeloma, liver cirrhosis, autoimmune conditions), and an inverted ratio (<1.0) warrants investigation. Higher A/G ratio in population studies is associated with better nutritional status and is inversely correlated with systemic inflammation; lower values are independently associated with greater frailty, shorter telomere length, higher all-cause mortality, and accelerated biological aging in several large cohort analyses. As a composite reflecting both hepatic synthetic function and the inflammatory immune response, the A/G ratio can add interpretive context to albumin and globulin assessed in isolation.

Alkaline phosphatase (ALP) is a hydrolase enzyme that cleaves phosphate groups at alkaline pH and is measured in serum as a composite of isoforms originating primarily from the liver, bone, intestine, and placenta. Elevated ALP most commonly reflects hepatobiliary disease — cholestasis in particular — or accelerated bone turnover as seen in Paget's disease, bone metastases, and hyperparathyroidism; concurrent elevation of GGT (and/or 5'-nucleotidase) alongside ALP points to a hepatic rather than osseous origin, since these enzymes are not produced by osteoblasts. Low ALP can indicate hypophosphatasia, zinc or magnesium deficiency, or hypothyroidism. In longevity contexts, persistently elevated ALP has been associated with higher all-cause and cardiovascular mortality in epidemiological cohorts, independent of liver disease status.

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are intracellular enzymes released into blood when hepatocytes are injured. ALT is relatively liver-specific, while AST also originates from muscle, heart, and red blood cells. Elevated values most often reflect metabolic-associated steatotic liver disease, alcohol use, viral hepatitis, or drug toxicity, and the AST/ALT ratio helps distinguish causes. Persistently raised liver enzymes are associated with cardiometabolic risk and higher all-cause mortality.

Apolipoprotein A-I is a 28-kDa hepatic and intestinal apolipoprotein that constitutes about 70% of total HDL protein and activates lecithin-cholesterol acyltransferase (LCAT), driving reverse cholesterol transport from peripheral tissues back to the liver. It is measured by immunonephelometric or immunoturbidimetric assay, calibrated against the WHO/IFCC SP1-01 reference material; typical reference ranges are 110-180 mg/dL in men and 120-200 mg/dL in women. In the INTERHEART case-control study of 12,461 cases and 14,637 controls across 52 countries, the ApoB/ApoA-I ratio was the strongest single lipid predictor of incident myocardial infarction (odds ratio approximately 3.25 for top vs bottom quintile), outperforming the conventional LDL-C/HDL-C ratio. Major confounders include acute infection and inflammation (acute-phase decreases of 20-30%), pregnancy, oral estrogens (raise it), severe hepatic disease, and rare ApoA-I mutations (Tangier disease, ApoA-I Milano).

Apolipoprotein B (ApoB) is the structural protein of atherogenic lipoproteins, including LDL, VLDL, IDL, and Lp(a). Because ApoB-100 is typically present as approximately one molecule per atherogenic particle, plasma ApoB serves as a clinical proxy for atherogenic particle number. Multivariable Mendelian randomization (e.g., Richardson et al.) prioritizes ApoB and particle burden as the causal lipid driver of coronary artery disease, making ApoB a more accurate predictor of cardiovascular events and mortality than LDL cholesterol alone. Lower ApoB is associated with reduced atherosclerotic disease. Per the NLA Expert Clinical Consensus on apoB, typical targets are <90 mg/dL for borderline-to-intermediate risk, <80 mg/dL for high risk, and <60 mg/dL for very high risk.

Apolipoprotein E (APOE) is a lipid-transport protein encoded by the APOE gene, which segregates into three alleles — ε2, ε3, and ε4 — producing six possible genotypes. The ε4 allele is the strongest common genetic risk factor for late-onset Alzheimer's disease, conferring roughly 3-fold higher risk per allele in heterozygous carriers, while homozygous ε4/ε4 carriers face approximately 10–15-fold elevated risk (Farrer 1997 meta-analysis); a 2024 study (Fortea et al., Nature Medicine) proposed reclassifying this genotype as a near-deterministic genetic form of late-onset Alzheimer's, though this remains under debate. Conversely, ε2 appears modestly protective against Alzheimer's and is associated with lower LDL levels. Because APOE genotype is fixed at birth and has large, well-replicated effect sizes, it is uniquely informative for stratifying lifetime dementia and cardiovascular risk, though penetrance is incomplete and lifestyle factors can modify the trajectory.

The AST/ALT ratio, also called the De Ritis ratio after Fernando De Ritis who first described its diagnostic utility in the 1950s, is calculated as aspartate aminotransferase (AST) divided by alanine aminotransferase (ALT). In hepatocellular injury, ALT tends to rise more than AST because it is more liver-specific, yielding a ratio below 1.0; in alcoholic liver disease the ratio typically exceeds 2.0, partly because alcohol-related mitochondrial damage preferentially depletes intracellular pyridoxal phosphate needed for ALT synthesis and because AST release from damaged mitochondria predominates. A ratio above 1.0 in the setting of elevated transaminases also raises suspicion for cirrhosis, where regenerative nodules lose cytosolic ALT preferentially. Beyond liver disease, an elevated AST/ALT ratio in individuals with normal or near-normal absolute enzyme values may reflect non-hepatic AST contributions from skeletal or cardiac muscle. In population-level longevity research, a ratio persistently above ~1.0 in the absence of acute illness or vigorous exercise is associated with higher cardiometabolic risk and all-cause mortality, and should prompt investigation for alcoholic liver injury, steatohepatitis, or underlying cirrhosis rather than being dismissed as unremarkable.

Bilirubin is the predominant end-product of haem catabolism, formed when reticuloendothelial cells degrade senescent red blood cells; it circulates as unconjugated (indirect) bilirubin bound to albumin before hepatic uptake, conjugation with glucuronic acid, and biliary excretion. Elevated total bilirubin is classified as pre-hepatic (haemolysis), hepatic (hepatocellular dysfunction), or post-hepatic (biliary obstruction) based on the unconjugated-to-conjugated ratio and accompanying enzymes. Mildly elevated unconjugated bilirubin in isolation — as in Gilbert syndrome — is generally benign and is associated with a more favourable cardiovascular risk profile in observational data, possibly via bilirubin's antioxidant properties as a lipid-soluble radical scavenger. Severely elevated bilirubin is a marker of advanced liver failure and a component of prognostic scores such as MELD.

C-peptide is the 31-amino-acid chain excised from proinsulin in pancreatic beta cells. One molecule is co-secreted per insulin molecule, so fasting C-peptide directly reflects endogenous insulin secretion. Negligible hepatic extraction gives it a half-life of ~30 min versus insulin's 3–5 min, keeping it measurable under exogenous insulin therapy. Fasting C-peptide serves two roles: as a marker of beta-cell reserve (low values signal secretory loss in type 1 and late-stage type 2 diabetes) and as an index of chronic hyperinsulinaemia (elevated values indicate insulin resistance and compensatory hypersecretion). An NHANES cohort (Patel et al. 2012; n = 5,153 non-diabetic adults) found fasting C-peptide outperformed fasting insulin and resistance indices in predicting cardiovascular and all-cause death. A meta-analysis of 23 studies (Ahmadirad et al. 2023) confirmed a 22% higher all-cause mortality risk (HR 1.22; 95% CI 1.12–1.32) and 38% higher cardiovascular mortality (HR 1.38; 95% CI 1.08–1.77). In type 1 diabetes, residual secretion is protective; in insulin-resistant individuals, elevation signals metabolic burden. Interpretation requires clinical context (fasting glucose, HbA1c, diabetes status); C-peptide is most informative alongside fasting insulin and glucose.

CA-125 (cancer antigen 125) is a high-molecular-weight mucin-like glycoprotein encoded by MUC16 and expressed on coelomic and Müllerian-derived epithelium; it is shed into the circulation in proportion to epithelial surface disruption. Serum CA-125 above 35 U/mL is used clinically as a tumour marker for epithelial ovarian cancer, particularly for monitoring response to treatment and detecting recurrence, though sensitivity for early-stage disease is limited to approximately 50%. Elevated CA-125 is non-specific: endometriosis, uterine fibroids, pelvic inflammatory disease, pleural effusion, and hepatic cirrhosis can all produce substantial elevations in the absence of malignancy. The multivariate Risk of Ovarian Malignancy Algorithm (ROMA), combining CA-125 with HE4, improves diagnostic specificity and is preferred over CA-125 alone for pre-operative risk stratification.

Cell-free DNA refers to short (typically 140-200 bp mono-nucleosome and 300-400 bp di-nucleosome) double-stranded DNA fragments released into plasma by apoptotic and necrotic cells. It is quantified by fluorometry (Qubit), qPCR or droplet digital PCR after column-based plasma extraction, with healthy adult plasma concentrations roughly 1-10 ng/mL. cfDNA rises with chronological age, chronic inflammation ("inflammageing"), strenuous exercise, sepsis, autoimmune disease, trauma and active malignancy, and is regarded as an aggregate readout of tissue turnover and immune activation. The 2018 Moss et al. methylation-deconvolution work showed that cell-type-specific methylation signatures allow tissue-of-origin mapping of cfDNA, enabling non-invasive quantification of cell death in specific organs. cfDNA forms the analytic substrate for liquid-biopsy oncology, transplant rejection monitoring, non-invasive prenatal testing and multi-cancer early detection. Major confounders are haemolysis during phlebotomy and delayed plasma separation.

The coronary artery calcium (CAC) score is a non-contrast cardiac CT measurement reported as an Agatston score that quantifies calcified atherosclerotic plaque as a marker of total atherosclerotic burden; non-calcified (soft) plaque is not detected. A higher CAC score is strongly associated with future myocardial infarction, cardiovascular events, and all-cause mortality, and primary-prevention guidelines (2018 ACC/AHA, 2021 ESC) recommend CAC for risk reclassification in intermediate-risk adults. A score of zero indicates very low short-term event risk but does not fully exclude atherosclerosis, particularly in younger adults or those with elevated Lp(a) or familial hypercholesterolemia. Standard Agatston categories are: 0 (very low), 1–99 (mild), 100–399 (moderate), and ≥400 (severe). The 2026 ACC/AHA Dyslipidemia Guideline integrates CAC with the PREVENT-ASCVD risk equations for refining statin decisions in borderline-risk adults.

Cortisol is the primary glucocorticoid of the adrenal cortex, the end-effector of the hypothalamic–pituitary–adrenal (HPA) axis. It follows a pronounced diurnal rhythm: levels surge 50–160 % within 30–45 minutes of awakening — the cortisol awakening response (CAR) — then fall to a nadir by late evening. Measurement uses morning serum or serial saliva; the CAR is captured at wake-up, +15 min, and +30 min. Chronic HPA overactivity — elevated evening cortisol, a flattened diurnal slope, or a blunted CAR — drives skeletal-muscle catabolism via glucocorticoid receptor–mediated suppression of IGF-1 signalling and ubiquitin–proteasome activation, accelerating sarcopenic muscle-mass loss. Lower overall diurnal output associates with longevity: in the Leiden Longevity Study (Noordam 2012), offspring of nonagenarian siblings showed significantly lower diurnal salivary cortisol than age-matched controls, independent of health behaviours. A scoping review by Palmese et al. (2025) in European Geriatric Medicine found consistent associations between a steeper diurnal drop and faster gait speed, better chair-rise time, and lower frailty risk, but the authors classify the evidence as fragmented and insufficient to establish causation. The link with cognitive aging is also associational: in the Whitehall II and NSHD cohorts (Tsui 2020, Neurology), a higher morning-to-evening ratio at mid-life prospectively linked to better later cognitive scores, while blunted variation predicted worse performance.

Creatine kinase (CK) is an enzyme that catalyses the reversible transfer of a phosphate group from phosphocreatine to ADP, regenerating ATP in tissues with high and fluctuating energy demands, principally skeletal muscle, cardiac muscle, and brain. Serum CK is released when these tissues are damaged, with isoenzyme fractionation — CK-MM (skeletal), CK-MB (cardiac), CK-BB (brain) — guiding organ-level attribution; historically CK-MB elevation was a diagnostic criterion for myocardial infarction, but since the 2018 Fourth Universal Definition of MI cardiac troponin (preferably high-sensitivity) has replaced CK-MB as the standard biomarker. Transient CK elevations after unaccustomed strenuous exercise are expected and benign, whereas persistent elevation suggests myopathy, rhabdomyolysis, or statin-induced muscle toxicity. In sarcopenia research, resting CK and its trajectory are explored as indirect indices of muscle membrane integrity.

Creatinine is a breakdown product of muscle creatine, produced at a relatively constant rate and cleared predominantly by glomerular filtration with a small contribution from tubular secretion, making serum creatinine a core marker of renal function. Because absolute values depend on muscle mass, age, sex, dietary meat intake, and creatine supplementation, laboratories report estimated glomerular filtration rate (eGFR) using the 2021 race-free CKD-EPI equation as current standard. Lower eGFR indicates reduced filtration capacity and is robustly associated with cardiovascular events, frailty, and mortality.

Cystatin C is a small (13 kDa) cysteine protease inhibitor produced at a constant rate by all nucleated cells, freely filtered by the glomerulus, and almost entirely reabsorbed and catabolized by the proximal tubule, making it a sensitive endogenous filtration marker. Unlike creatinine, its serum concentration is minimally influenced by muscle mass, sex, or dietary protein, though it rises with higher body fat, thyroid dysfunction, and corticosteroid use, which should be considered in interpretation. The 2021 race-free CKD-EPI creatinine and combined creatinine–cystatin C (eGFRcr-cys) equations provide more accurate eGFR estimates (the cystatin C–only equation in current use is still the 2012 form, which was already race-free) in populations where creatinine is confounded by muscle mass extremes. Elevated cystatin C is a robust predictor of cardiovascular events, cognitive decline, and all-cause mortality, and epidemiological analyses suggest it may track true renal aging more faithfully than creatinine-derived eGFR.

D-dimer is a cross-linked fibrin degradation product generated when plasmin cleaves stabilised fibrin, and serves as a global marker of activated coagulation and fibrinolysis. It is measured by latex-enhanced immunoturbidimetric or ELISA-based assays, reported either in fibrinogen-equivalent units (FEU) or D-dimer units (DDU). Its principal clinical role is ruling out venous thromboembolism: a value below the assay-specific cutoff (commonly <500 ng/mL FEU) combined with a low pre-test probability has a negative predictive value above 95%. The ADJUST-PE study (Righini 2014) validated an age-adjusted cutoff (age x 10 ng/mL FEU for patients >50 years), which safely increases specificity in older adults. D-dimer rises physiologically with age, pregnancy, inflammation, surgery, trauma, malignancy, sepsis, and severe liver or kidney disease, so a positive result is non-specific and must be interpreted alongside imaging.

Dehydroepiandrosterone sulfate (DHEA-S) is the sulfated, long-circulating form of DHEA, secreted by the adrenal cortex (zona reticularis) and serving as a precursor to androgens and estrogens in peripheral tissues. Although it has minor diurnal variation, serum levels are far more stable than DHEA itself, making DHEA-S the preferred clinical marker of adrenal androgen output. It peaks in early adulthood and declines steeply with age (adrenopause); lower values are observationally associated with frailty, reduced bone density, and impaired immune function, though DHEA supplementation trials have largely been null for hard outcomes.

Estradiol (E2) is the most biologically active estrogen, produced mainly in the ovaries before menopause and in smaller amounts via aromatization of androgens in adipose tissue, brain, bone, liver, breast, and other peripheral tissues. It supports endothelial function, bone turnover, reproductive tissues, and may support cognition. After menopause, levels drop sharply and remaining production occurs via peripheral aromatization, contributing to accelerated bone loss, vasomotor symptoms, and a multifactorial rise in cardiometabolic risk. In men, modest estradiol from aromatization is important for bone health.

F2-isoprostanes are a family of prostaglandin F2α-like compounds formed non-enzymatically by free radical-mediated peroxidation of arachidonic acid esterified in phospholipids; they are released into plasma and excreted in urine, where 8-iso-PGF2α (iPF2α-III) is the most widely measured species. Because their production depends on the rate of in vivo lipid peroxidation and is independent of dietary fatty acid intake at typical measurement conditions, urinary F2-isoprostanes are regarded as the most reliable in vivo biomarker of systemic oxidative stress. Elevated levels are consistently found in cigarette smokers, obese individuals, diabetics, and patients with cardiovascular disease or neurodegenerative disorders; values decrease with antioxidant-rich diets, caloric restriction, and exercise. In epidemiological studies, higher F2-isoprostane levels predict incident cardiovascular events and are associated with accelerated biological aging.

Fasting glucose is the plasma blood-sugar concentration after at least eight hours without caloric intake. It reflects baseline glucose homeostasis driven by hepatic glucose output, β-cell insulin secretion, peripheral insulin sensitivity, and counter-regulatory hormones such as glucagon, with renal glucose handling as a further contributor. Persistently elevated values indicate impaired fasting glucose, prediabetes, or type 2 diabetes, and Mendelian randomization supports a causal effect of higher fasting glucose on coronary disease. Even within the upper-normal range, rising fasting glucose tracks with increased risk, so stable lower-normal values are generally favorable.

Fasting insulin measures circulating insulin after an overnight fast and reflects β-cell output, hepatic insulin clearance, and peripheral insulin sensitivity together. Elevated fasting insulin is one of the earliest signs of insulin resistance and often appears before fasting glucose or HbA1c rise, consistent with the natural history seen in cohorts such as Whitehall II and IRAS. Hyperinsulinemia is associated with metabolic syndrome, type 2 diabetes, and a higher risk of cardiovascular disease and all-cause mortality. Lower fasting insulin generally indicates better insulin sensitivity and metabolic flexibility.

Ferritin is a ubiquitous intracellular iron-storage protein that releases a small fraction into circulation; serum ferritin is therefore the most widely used biomarker for assessing total body iron stores. Low ferritin (typically <30 µg/L, or <15 µg/L in some guidelines) indicates depleted iron stores preceding overt iron-deficiency anemia and is associated with fatigue, reduced exercise capacity, impaired cognitive function, and hair loss even before hemoglobin falls. Conversely, elevated ferritin is a potent positive acute-phase reactant that rises substantially with inflammation, infection, liver disease, obesity, and metabolic syndrome, complicating interpretation; persistently elevated values without these confounders may reflect hereditary hemochromatosis or secondary iron overload, which accelerates oxidative stress, hepatic fibrosis, and cardiomyopathy. Optimal ferritin for health is debated; many longevity-oriented practitioners target approximately 70–120 µg/L, with caution warranted at both extremes.

Fibrinogen (coagulation factor I) is a hepatically produced 340-kDa glycoprotein that polymerises to fibrin during clot formation and is also an acute-phase reactant. It is measured by the Clauss functional assay (clot-based) or immunoassay; typical reference range is 200-400 mg/dL (2.0-4.0 g/L). Beyond its role in haemostasis, plasma fibrinogen is an independent cardiovascular risk marker: the Fibrinogen Studies Collaboration's individual-participant meta-analysis of 154,211 adults (JAMA 2005) demonstrated a hazard ratio of approximately 2.4 (95% CI 2.3–2.6) for coronary heart disease per 1 g/L higher usual fibrinogen, with similar associations for stroke and non-vascular mortality. Major confounders include acute infection or trauma (rise within 24-48 h), smoking, pregnancy, oral estrogens, malignancy, and chronic inflammation; values can be reduced in disseminated intravascular coagulation, severe liver disease, fibrinolytic therapy and congenital hypofibrinogenaemia.

Free T3 (fT3) and free T4 (fT4) are the unbound, biologically active fractions of triiodothyronine and thyroxine. T4 is the main thyroid secretion product and is deiodinated peripherally to the more potent T3, which acts on nuclear receptors to regulate metabolism, thermogenesis, and cardiovascular function. Measuring free fractions avoids interference from binding-protein changes and helps distinguish primary thyroid disease, central hypothyroidism, and non-thyroidal illness when interpreted alongside TSH.

Free testosterone is the 1-4% of circulating testosterone not bound to SHBG or albumin and represents the immediately bioactive fraction. The gold-standard analytical method is equilibrium dialysis followed by LC-MS/MS; the most validated alternative is the calculated free testosterone (cFT) derived from total testosterone, SHBG and albumin using the Vermeulen equation. Direct analogue immunoassays are inaccurate and not recommended by the Endocrine Society. According to the 2018 Endocrine Society guideline, a calculated free testosterone below approximately 65 pg/mL (220 pmol/L) in symptomatic men supports a diagnosis of androgen deficiency, with the lower limit of normal in healthy young men typically around 70 pg/mL (~7 ng/dL, 0.24 nmol/L). Confounders include SHBG-altering states (insulin resistance, thyroid disease, oral estrogens), morning vs afternoon sampling, acute illness, and inter-assay differences in SHBG that propagate into the calculation.

Fructosamine denotes glycated serum proteins — chiefly albumin — formed via non-enzymatic glucose binding and Amadori rearrangement to a stable ketoamine. Albumin's ~14–21-day turnover yields a 2–3-week glucose window versus HbA1c's 8–12 weeks. Reference values: 200–285 µmol/L. Its primary role is substituting for HbA1c when red-blood-cell turnover is abnormal: haemolytic anaemia, sickle-cell disease, iron- or B12-deficiency anaemia, erythropoietin-treated CKD, and pregnancy all distort HbA1c. Results are unreliable below serum albumin 3.0 g/dL (cirrhosis, nephrotic syndrome, protein-losing enteropathy); separately assayed glycated albumin is then preferred. In ARIC (Selvin et al., 2014; n ≈ 12,300; up to 20 years), elevated fructosamine predicted incident diabetes and microvascular complications comparably to HbA1c; associations with retinopathy and CKD persisted after HbA1c adjustment. AMORIS (Malmström et al., 2015; n = 338,443; median 19 years) found MI and all-cause mortality risk rising at ≥2.30 mmol/L; the highest tier (≥2.70 mmol/L) yielded HR 2.88 for MI and 2.31 for all-cause mortality after adjustment for cardiovascular risk factors and fasting glucose. In octogenarians without diabetes, albumin-corrected fructosamine predicted all-cause mortality (HR 1.27 per 1-µmol/g; Zhou et al., 2022, Healthy Ageing and Biomarkers Cohort Study). Fructosamine assays lack cross-laboratory standardisation, and an independent causal contribution to cardiometabolic risk beyond the hyperglycaemia they reflect has not been established in intervention trials.

Galectin-3 is a 30-kDa beta-galactoside-binding lectin secreted by activated macrophages that drives myocardial fibroblast proliferation, collagen deposition and adverse cardiac remodelling. It is measured in plasma or serum by an FDA-cleared ELISA or Vidas/Architect chemiluminescent immunoassay; multicentre validation (Christenson 2010) reported 90th, 95th and 97.5th percentile cutoffs of approximately 17.6, 20.3 and 22.1 ng/mL in apparently healthy adults, and de Boer 2011 reported that levels above 17.8 ng/mL are associated with higher heart-failure mortality. The 2017 ACC/AHA/HFSA focused update gave galectin-3 a Class IIb recommendation for additive risk stratification, but the 2022 AHA/ACC/HFSA heart-failure guideline (Heidenreich et al.) and the 2021 ESC HF guideline downgraded routine galectin-3 measurement relative to natriuretic peptides; current use is limited to research and select prognostic settings. Major confounders include age (rises modestly), chronic kidney disease (eGFR is the strongest non-cardiac determinant), pulmonary fibrosis, hepatic fibrosis, and active malignancy; assay platform differences mean serial measurements should use the same method.

Glial fibrillary acidic protein is the principal intermediate filament of mature astrocytes and a marker of reactive astrogliosis. Ultrasensitive immunoassays allow measurement in plasma, where concentrations rise across the Alzheimer disease continuum and are detectable in preclinical, amyloid-positive but cognitively unimpaired individuals. Plasma GFAP discriminates amyloid-PET positive from negative status with higher accuracy than cerebrospinal fluid GFAP and is being evaluated as a complement to amyloid- and tau-specific markers within the ATN framework, where it is sometimes incorporated as an inflammation or astrocyte axis. GFAP also rises in traumatic brain injury, where it is used clinically alongside UCH-L1 for triage. It is not Alzheimer-specific and should be interpreted alongside p-tau217 and amyloid biomarkers.

Gamma-glutamyl transferase (GGT) is a membrane-bound enzyme that transfers gamma-glutamyl groups and supports glutathione recycling, with highest activity in liver, biliary epithelium, and kidney. Serum GGT rises with cholestasis, alcohol intake, hepatic steatosis, and many enzyme-inducing drugs, making it a sensitive but unspecific liver marker. Beyond hepatology, higher GGT within the reference range is independently associated with insulin resistance, cardiovascular disease, and increased all-cause mortality, marking oxidative stress.

GlycA is a composite nuclear magnetic resonance (NMR) spectroscopy signal arising predominantly from N-acetyl methyl groups on acute-phase glycoproteins — primarily α1-acid glycoprotein, α1-antitrypsin, haptoglobin, α1-antichymotrypsin, and transferrin — and reflects the integrated concentration and glycosylation state of these inflammatory proteins. Because it integrates across multiple acute-phase reactants simultaneously, GlycA has lower within-person variability than hs-CRP and may better capture chronic low-grade systemic inflammation rather than acute fluctuations. Population studies associate higher GlycA with incident cardiovascular disease, type 2 diabetes, non-alcoholic fatty liver disease, cancer, and all-cause mortality, with some evidence that it predicts these outcomes independently of and complementary to hs-CRP. GlycA is currently a research-grade marker not routinely available in clinical laboratories and is most often reported as part of NMR lipoprotein panels.

HDL cholesterol (high-density lipoprotein cholesterol) measures the cholesterol carried by HDL particles, which transport cholesterol from tissues back to the liver in reverse cholesterol transport. Population studies historically associated higher HDL-C with lower cardiovascular risk, but Mendelian randomization and HDL-raising drug trials show the relationship is non-causal and U-shaped: very high HDL-C is also linked to increased mortality. HDL functionality matters more than its concentration alone.

High-sensitivity troponin (hs-Tn) assays measure the cardiac isoforms troponin I (hs-TnI) or troponin T (hs-TnT) at concentrations about 10-fold lower than conventional assays, with analytical coefficient of variation ≤10% at the 99th-percentile reference limit. This sensitivity enables detection of the small troponin leaks that occur with myocardial injury from acute MI, myocarditis, takotsubo syndrome, and demand ischemia, as well as low-grade chronic cardiomyocyte injury. Beyond acute chest-pain rule-out, chronically elevated hs-Tn in the population predicts incident heart failure, atrial fibrillation, and cardiovascular and all-cause mortality independently of established risk factors, making it an emerging biomarker of subclinical cardiac aging and injury. Values must always be interpreted in clinical context, as non-cardiac causes (e.g., renal failure, sepsis, pulmonary embolism) can also elevate troponin.

Homocysteine is a sulfur-containing amino acid produced during methionine metabolism and cleared via remethylation or transsulfuration pathways that depend on folate, vitamin B12, and vitamin B6. Elevated plasma homocysteine reflects impaired one-carbon metabolism and is associated with vascular endothelial dysfunction, atherosclerosis, and stroke; cognitive decline and dementia are observationally associated, but causality is uncertain. Major randomized trials of B-vitamin lowering (HOPE-2, NORVIT, VISP, SEARCH, VITATOPS) have not consistently reduced cardiovascular events overall, though some meta-analyses suggest a small reduction in stroke risk. Homocysteine is therefore interpreted as a risk and metabolic-health marker.

High-sensitivity C-reactive protein (hs-CRP) is a liver-produced acute-phase protein, induced primarily by IL-6, and measured with an assay sensitive enough to detect low-grade systemic inflammation. Outside acute infection, persistently elevated hs-CRP signals chronic inflammation linked to atherosclerosis, insulin resistance, and metabolic syndrome. hs-CRP is a downstream marker of IL-6–driven inflammation; Mendelian randomization does not support CRP itself as causal for coronary heart disease, while trials targeting upstream inflammation (e.g., CANTOS with canakinumab) reduce cardiovascular events. It remains a useful marker of inflammatory burden associated with cardiovascular and mortality risk.

Insulin-like growth factor 1 (IGF-1) is produced mainly in the liver under growth hormone stimulation and mediates many anabolic GH effects on muscle, bone, and other tissues. As a biomarker it serves as a stable surrogate for GH activity and reflects nutritional status and protein intake. Higher levels in adulthood are linked to greater cancer risk; very low levels are observationally associated with frailty and reduced lean mass, though this association may partly reflect underlying disease or malnutrition rather than a direct causal longevity detriment. The net relationship between IGF-1 and longevity in humans is complex and not simply U-shaped.

Interleukin-6 (IL-6) is a pleiotropic cytokine produced by immune cells, adipocytes, endothelial cells, and senescent cells that signals through the membrane-bound IL-6 receptor (classical signalling) or through soluble IL-6 receptor trans-signalling, with distinct tissue effects. Acutely, IL-6 is a key inducer of the hepatic acute-phase response — driving CRP, fibrinogen, and serum amyloid A synthesis — and plays an essential role in host defence; after vigorous exercise, skeletal muscle transiently secretes high amounts, reflecting a myokine function. Chronically elevated circulating IL-6, as seen in chronic inflammatory conditions, obesity, and biological aging (inflammaging), is an independent predictor of all-cause and cardiovascular mortality across multiple large prospective cohorts, with prognostic value that remains significant after adjusting for CRP. In longevity research, IL-6 is a central biomarker of inflammaging and a mechanistic target under investigation, given that IL-6 elevation drives downstream pathways linked to sarcopenia, cognitive decline, frailty, and cancer progression.

Ionized calcium (iCa²⁺), also called free calcium, is the biologically active fraction of total serum calcium, comprising approximately 45–50% of the total and not bound to albumin or complexed with anions such as phosphate and citrate. It is maintained within a narrow range (1.15–1.35 mmol/L) by a tightly coupled hormonal axis involving PTH, calcitriol, and calcitonin acting on bone, kidney, and intestine. Ionized calcium measurement is preferred over total calcium in settings where albumin is abnormal — as in critical illness, liver disease, or hypoalbuminaemia — because total calcium corrected by simple albumin formulas can be unreliable. Hypercalcaemia raises suspicion for primary hyperparathyroidism or malignancy, while hypocalcaemia presents with neuromuscular irritability and, if severe, arrhythmia.

LDL cholesterol (low-density lipoprotein cholesterol) reflects the cholesterol carried by LDL particles in the bloodstream. Excess ApoB-containing LDL particles can enter and be retained in the arterial intima, where modification (e.g., oxidation) and inflammation drive atherosclerotic plaque formation. LDL-C is a long-established causal risk factor for atherosclerotic cardiovascular disease, heart attack, and stroke; lifelong genetically lower LDL-C (e.g., PCSK9 loss-of-function variants) reduces ASCVD risk dose-dependently. From a longevity perspective LDL-C is lower-is-better, ideally interpreted alongside ApoB and Lp(a), as LDL-C and ApoB can be discordant. Current reference documents include the 2025 ESC/EAS Focused Update and the 2026 ACC/AHA Dyslipidemia Guidelines.

LDL-P, or LDL particle number, quantifies the total concentration of low-density lipoprotein particles in plasma rather than their cholesterol payload, typically measured by nuclear magnetic resonance (NMR) spectroscopy or ion-mobility analysis. Because each LDL particle carries one apolipoprotein B-100 molecule, LDL-P is conceptually equivalent to (and strongly correlated with) the apoB-bearing atherogenic particle count; both reflect the atherogenic particle burden available for subendothelial retention, though units differ (LDL-P in nmol/L vs apoB in mg/dL) and apoB additionally counts VLDL/IDL/Lp(a) particles. In discordance analyses — where LDL-C and LDL-P diverge, most commonly in the setting of hypertriglyceridaemia, insulin resistance, or low HDL — LDL-P is consistently the stronger predictor of cardiovascular events, supporting the concept that it is particle number rather than cholesterol mass per particle that drives atherosclerosis risk. Reference intervals are typically reported as nmol/L, with higher cardiovascular risk associated with LDL-P above approximately 1,000–1,200 nmol/L.

Lp-PLA2 (platelet-activating factor acetylhydrolase) is a calcium-independent enzyme secreted mainly by macrophages and lymphocytes; in plasma roughly 80% circulates bound to LDL, where it hydrolyses oxidised phospholipids on retained LDL particles to release lysophosphatidylcholine and oxidised fatty acids, both proinflammatory in the arterial wall. It is measured by mass (immunoassay, the FDA-cleared PLAC test, with values above approximately 200-225 ng/mL considered elevated) or by activity assay (cutoff approximately 225 nmol/min/mL). The Lp-PLA2 Studies Collaboration meta-analysis (Thompson 2010, 79,036 participants) reported a risk ratio of 1.10-1.11 per standard deviation increment for coronary heart disease and ischaemic stroke, comparable to non-HDL cholesterol. Major confounders include statins (lower Lp-PLA2 by approximately 20-30%), fibrates, ezetimibe, weight loss, and the rare loss-of-function PLA2G7 V279F variant common in East Asian populations.

Lipoprotein(a) is an LDL-like particle in which apolipoprotein(a) is covalently linked to apoB-100 via a disulfide bond. Plasma levels are largely (often cited around 70–90%) genetically determined and usually stable across adulthood, although modest changes can occur with menopause, kidney disease, or inflammation. Mendelian randomization at the LPA locus supports a causal role for elevated Lp(a) in myocardial infarction, ischemic stroke, and calcific aortic valve stenosis. Because diet and lifestyle barely influence it, Lp(a) should be measured at least once to stratify lifetime cardiovascular risk. Per EAS 2022 thresholds, rule-in risk levels are >50 mg/dL (>125 nmol/L) and rule-out levels are <30 mg/dL (<75 nmol/L).

The absolute lymphocyte count (ALC) is the total number of circulating lymphocytes — comprising T cells, B cells, and NK cells — derived from a complete blood count differential. In adults, the normal range is approximately 1.0–4.0 × 10⁹/L. Lymphopenia (ALC <1.0 × 10⁹/L) is a recognized marker of immune deficiency and occurs in HIV infection, autoimmune diseases, lymphoma, following chemotherapy or radiotherapy, and in severe malnutrition. With aging, the absolute count tends to decline and the composition shifts — naïve T cells decrease while memory and effector cells accumulate — a constellation termed immunosenescence. Population studies consistently link lower lymphocyte count with higher all-cause mortality, greater frailty, and increased infection vulnerability, and ALC is a component of several validated biological aging indices. Importantly, lymphopenia was a strong predictor of severe COVID-19, underscoring its broader role as a marker of immune reserve.

Serum magnesium reflects the small circulating fraction of total body magnesium, with roughly 99% stored in bone, muscle, and soft tissue, making serum levels an insensitive indicator of intracellular magnesium status. Magnesium is an essential cofactor for hundreds of enzymatic reactions including ATP synthesis, DNA replication and repair, and serves as a physiological calcium channel antagonist involved in neuromuscular transmission and cardiac rhythm. Hypomagnesaemia (typically below 0.75 mmol/L) is associated with cardiac arrhythmias, neuromuscular excitability, insulin resistance, and increased vascular calcification; it is common in type 2 diabetes, chronic alcohol use, and with prolonged use of proton pump inhibitors or loop diuretics. Epidemiological data link lower habitual magnesium intake and lower serum levels with higher all-cause mortality and accelerated biological aging.

Multi-cancer early detection (MCED) tests are blood assays interrogating plasma cell-free DNA - chiefly methylation patterns and fragmentomics - to detect a shared "cancer signal" across many tumour types and predict the tissue of origin. The most advanced is GRAIL's Galleri, which uses targeted methylation sequencing across more than 100,000 genomic regions. In the PATHFINDER cohort study (Schrag 2023), Galleri detected a cancer signal in 1.4% of 6,621 adults aged >=50, with specificity 99.1% and a positive predictive value of 38%; the underlying case-control CCGA validation (Klein 2021) reported specificity 99.5%, tissue-of-origin accuracy 88%, and overall sensitivity of about 51%, rising sharply with stage. SYMPLIFY (Nicholson 2023) showed a positive predictive value of 75.5% in symptomatic primary-care patients. Galleri is marketed in the US as a laboratory-developed test; GRAIL submitted a PMA to the FDA on 29 January 2026 (informed by PATHFINDER 2, presented at ESMO October 2025), and headline NHS-Galleri results (February 2026) missed the primary stage-shift endpoint but showed fewer stage-IV diagnoses in a pre-specified subgroup of 12 deadly cancers. Confounders include benign inflammation, recent biopsy, pregnancy and clonal haematopoiesis.

Myeloperoxidase is a heme-containing peroxidase stored in azurophilic granules of neutrophils and monocytes that generates hypochlorous acid and other reactive oxidants during inflammation. Released into plasma at sites of vascular inflammation and plaque rupture, MPO is a marker of leukocyte activation, oxidative stress and atherosclerotic plaque instability. Plasma MPO is typically measured by sandwich ELISA or chemiluminescent immunoassay, with healthy reference values commonly below 350-630 pmol/L (assay-dependent). In Brennan et al. (NEJM 2003), among 604 patients with chest pain and a negative initial troponin, baseline plasma MPO independently predicted myocardial infarction and major adverse cardiovascular events at 30 days and 6 months. Confounders include any acute infection, ANCA-associated vasculitis (MPO is the autoantigen of p-ANCA), heparin therapy (which releases endothelial-bound MPO and falsely raises levels), smoking, and pregnancy.

The neutrophil-lymphocyte ratio (NLR) is derived from a standard complete blood count as the absolute neutrophil count divided by the absolute lymphocyte count, with a typical reference range of approximately 0.78–3.53 in healthy adults (Forget 2017). It reflects the balance between the innate (neutrophil-driven) and adaptive (lymphocyte-driven) arms of the immune system: higher NLR indicates relative dominance of innate proinflammatory activity and/or suppression of adaptive immunity. Elevated NLR is associated with worse prognosis in a broad range of cancers, higher incidence of cardiovascular events, chronic kidney disease, and metabolic syndrome, and independently predicts all-cause mortality in large population cohorts. Physiologically, NLR rises sharply during acute stress, infection, or corticosteroid use, but persistently elevated values at rest are a marker of chronic inflammaging and immune senescence, making it a useful, cost-free index derived from routine laboratory data.

Neurofilament light chain is a cytoskeletal protein of large myelinated axons released into cerebrospinal fluid and blood upon neuroaxonal injury. Ultrasensitive single-molecule assays such as Simoa (Quanterix) made serum and plasma NfL quantifiable at picogram-per-millilitre concentrations, transforming it into a disease-agnostic marker of axonal damage. Concentrations rise with age and across multiple sclerosis, amyotrophic lateral sclerosis, Alzheimer disease, frontotemporal dementia, traumatic brain injury and stroke. In multiple sclerosis it tracks relapse activity and treatment response; in ALS it correlates with progression and survival; in Alzheimer disease it indexes neurodegeneration within the ATN(N) framework. NfL is non-specific for aetiology and must be interpreted with age-adjusted reference values, but longitudinal change within an individual carries prognostic information.

Non-HDL cholesterol (non-HDL-C) equals total cholesterol minus HDL and captures the cholesterol in all atherogenic lipoproteins — LDL, VLDL, IDL, chylomicron remnants, and Lp(a). Because each carries apolipoprotein B (ApoB), non-HDL-C serves as a surrogate for total ApoB particle burden without a direct ApoB assay, and — unlike calculated LDL-C — is reliable in a non-fasting state. It is especially informative in elevated triglycerides, insulin resistance, or type 2 diabetes, where VLDL and remnant particles drive atherogenic risk yet are invisible to LDL-C. A 2012 JAMA meta-analysis by Boekholdt et al. pooled data from 8 statin trials (62,154 participants; 38,153 statin-treated) and found on-treatment non-HDL-C more strongly associated with cardiovascular events than on-treatment LDL-C. Patients reaching LDL-C below 100 mg/dL but non-HDL-C above 130 mg/dL retained significantly elevated residual risk. The 2019 ESC/EAS dyslipidaemia guidelines set non-HDL-C as a secondary target — below 85 mg/dL in very-high-risk and below 100 mg/dL in high-risk patients — 30 mg/dL above corresponding LDL-C goals, reflecting average VLDL cholesterol in the general population. Evidence is observational and from secondary analyses of intervention trials; causal direction is supported by Mendelian randomization and ApoB genetics, though whether targeting non-HDL-C over LDL-C yields incremental benefit in primary prevention remains open.

N-terminal pro-B-type natriuretic peptide (NT-proBNP) is the biologically inactive N-terminal fragment cleaved from proBNP when cardiomyocytes are stretched by elevated ventricular wall stress or volume overload. It has a longer half-life (~60–120 minutes) than BNP itself, making it analytically more stable. NT-proBNP is a central diagnostic and prognostic biomarker for heart failure: in non-acute settings, a value above 125 pg/mL is widely used as a rule-out threshold, while 2023 ESC HFA guidance recommends age-stratified rule-in thresholds for the non-acute / community setting (125 pg/mL for age <50, 250 pg/mL for 50–74 years, and 500 pg/mL for ≥75 years; acute-setting rule-in thresholds — 450/900/1800 pg/mL — are higher). Beyond established heart failure, modestly elevated NT-proBNP in community populations is independently associated with atrial fibrillation, coronary artery disease, chronic kidney disease, and all-cause mortality, so it is increasingly used as a subclinical cardiac stress marker in longevity assessments.

The Omega-3 index is a blood biomarker defined as the sum of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) expressed as a percentage of total fatty acids in red blood cell (erythrocyte) membranes. Because erythrocytes have a lifespan of approximately 120 days, the index reflects average long-term omega-3 intake over roughly two to four months — an advantage over plasma measurements, which capture only recent dietary exposure. Harris and Von Schacky (2004) proposed the index as a novel risk factor for coronary heart disease mortality, defining a desirable zone of ≥8% and a high-risk zone of ≤4%; most adults in Western populations fall between 4% and 6%. Multiple prospective cohort studies have reported inverse associations between the Omega-3 index and cardiovascular and all-cause mortality, independent of conventional lipid risk factors; a 2007 review by Harris synthesized this evidence and strengthened the case for clinical utility. The index is analytically standardized through a certified laboratory methodology, but it remains a risk marker rather than a risk factor in the causal sense — omega-3 supplementation trials have yielded heterogeneous cardiovascular outcomes, so the association cannot yet be interpreted as conclusively causal in all populations.

Oxidized LDL (oxLDL) refers to LDL particles in which the polyunsaturated fatty acids and apolipoprotein B-100 have undergone oxidative modification, typically within the subendothelial space where LDL is retained and exposed to reactive oxygen species and lipoxygenases. oxLDL is not recognised by the LDL receptor but is avidly taken up by macrophage scavenger receptors (SR-A, CD36), driving foam cell formation — the hallmark of early atherosclerotic lesions. Circulating oxLDL, measured by immunoassays using antibodies such as 4E6 or E06 that recognise oxidised phosphatidylcholine epitopes, is elevated in subjects with coronary artery disease, metabolic syndrome, and type 2 diabetes. As a direct indicator of lipid oxidation at the vessel wall, oxLDL is conceptually distinct from F2-isoprostanes (a systemic oxidative stress index) and from LDL-C or LDL-P, which do not reflect oxidative modification.

p-tau217 is tau protein phosphorylated at threonine 217 and the most Alzheimer-specific plasma biomarker currently available. Concentrations rise early along the amyloid cascade, before cognitive symptoms emerge, and correlate tightly with amyloid-PET and tau-PET burden. In head-to-head comparisons plasma p-tau217 outperforms p-tau181, p-tau231 and several MRI-based markers for discriminating Alzheimer disease from other neurodegenerative disorders. Two-step workflows using plasma p-tau217 reduce the need for amyloid-PET or cerebrospinal fluid testing when triaging patients with cognitive impairment for anti-amyloid therapy. Pre-analytical handling, assay platform and reference ranges still vary across vendors. The first FDA-cleared blood test for Alzheimer disease (Fujirebio Lumipulse G pTau217/β-Amyloid 1-42 plasma ratio, May 2025) uses a plasma p-tau217 ratio; Roche received an EU CE mark for the Elecsys pTau217 plasma assay on 12 May 2026.

Serum inorganic phosphate reflects the balance between intestinal absorption, renal tubular reabsorption, and exchange with bone and intracellular stores, regulated principally by parathyroid hormone (PTH), fibroblast growth factor 23 (FGF-23), and calcitriol. Hyperphosphataemia is clinically significant mainly in chronic kidney disease, where impaired renal phosphate excretion drives secondary hyperparathyroidism, vascular calcification, and cardiovascular mortality. Even within the normal reference range, higher serum phosphate in community-dwelling adults has been independently associated with left ventricular hypertrophy, arterial stiffness, and increased all-cause mortality in large prospective studies. Hypophosphataemia can accompany refeeding syndrome, vitamin D deficiency, or overactive FGF-23 signalling, impairing ATP generation, red cell function, and bone mineralisation.

Prostate-specific antigen (PSA) is a serine protease of the kallikrein family produced almost exclusively by prostatic epithelial cells; it liquefies seminal coagulum but leaks into the bloodstream in proportion to the degree of disruption of prostatic glandular architecture. Elevated serum PSA (commonly flagged above 4 ng/mL, though thresholds vary by age and risk profile) can reflect prostate cancer, benign prostatic hyperplasia, prostatitis, or recent mechanical stimulation, and has a positive predictive value for cancer of roughly 20–30% at levels of 4–10 ng/mL, though this estimate is substantially modified by age, race, DRE findings, and PSA density. Derived metrics — PSA density, PSA velocity, free-to-total PSA ratio, and age-specific ranges — improve specificity and reduce unnecessary biopsy. PSA-based screening has shown a modest reduction in prostate cancer mortality in some randomised trials (notably ERSPC) but not others (PLCO), and detects considerable indolent disease, leading to shared decision-making recommendations (USPSTF Grade C for ages 55–69) about its use as a longevity screening tool.

Red cell distribution width (RDW) is a quantitative measure of the variability in erythrocyte volume (anisocytosis), reported as the coefficient of variation of the erythrocyte volume distribution from automated hematology analyzers; the normal range is approximately 11.5–14.5%. Elevated RDW reflects heterogeneous red cell populations and arises from iron, B12, or folate deficiency, hemolysis, blood transfusion, or ineffective erythropoiesis. Independent of its role in anemia workup, higher RDW is a robust, reproducible predictor of all-cause and cardiovascular mortality in community populations, with associations observed even within the reference range. Proposed mechanisms include chronic inflammation, oxidative stress, nutritional deficiency, and impaired bone marrow function, all of which impair erythropoietic homeostasis. RDW is increasingly recognized as a simple, inexpensive marker of biological aging and physiological reserve.

Remnant cholesterol is the cholesterol content of triglyceride-rich lipoprotein (TRL) remnants — primarily VLDL and IDL in the fasting state, and chylomicron remnants postprandially. It is calculated as total cholesterol minus LDL-C minus HDL-C, avoiding specialised assays, though direct measurement yields higher absolute values. TRL remnants below ~70 nm traverse the arterial endothelium by active transcytosis, accumulate in the intima, and trigger foam-cell formation and endothelial dysfunction; each particle carries up to four times the cholesterol load of an LDL particle. Mendelian randomization in the Copenhagen General Population Study (Varbo et al., 2013 — 73,513 participants) estimated a causal odds ratio of 2.8 (95% CI 1.9–4.2) per 1 mmol/L genetic increase in nonfasting remnant cholesterol for ischemic heart disease, independent of HDL-C. Genetic instruments targeting TRIB1, GCKR, and APOA5 reinforce this causal inference and explain much of the residual cardiovascular risk in statin-treated patients with well-controlled LDL-C. Optimal fasting remnant cholesterol is generally placed below 0.5 mmol/L (~19 mg/dL); levels at or above 1.5 mmol/L (~58 mg/dL) are associated with ~1.9- to 2.3-fold higher myocardial infarction hazard in prospective Copenhagen cohort data. Because TRL metabolism worsens with insulin resistance and visceral adiposity — conditions linked to accelerated biological aging — elevated remnant cholesterol may partly mediate excess cardiovascular mortality in metabolic aging phenotypes.

Reverse T3 (3,3',5'-triiodothyronine) is the biologically inactive isomer of T3 produced when 5-deiodinase type 3 (D3) cleaves the inner ring of T4 instead of the outer ring. Plasma rT3 is measured by radioimmunoassay or LC-MS/MS, with typical reference values around 10-24 ng/dL (0.15-0.37 nmol/L). Levels rise sharply in non-thyroidal illness ("euthyroid sick syndrome"), starvation, severe trauma, sepsis, glucocorticoid, amiodarone or propranolol therapy, and during hepatic dysfunction. Although popular in functional medicine for diagnosing "rT3 dominance" or guiding T3 therapy, the American Thyroid Association, AACE and Endocrine Society do not recommend routine rT3 measurement; reviews and consensus statements conclude there is no validated clinical role for it outside research and rare presentations of consumptive hypothyroidism or thyroid hormone resistance. The T3/rT3 ratio is similarly unvalidated for healthy outpatients.

SHBG is a liver-synthesised glycoprotein that binds approximately 98% of circulating testosterone and 30-60% of estradiol with high affinity, regulating the bioavailable hormone fraction. It is measured by automated immunoassay (electrochemiluminescence, chemiluminescence) with platform-dependent reference ranges of roughly 10-80 nmol/L in adult men and 20-130 nmol/L in non-pregnant women; mass spectrometry serves as criterion method. SHBG falls with insulin resistance, obesity, type 2 diabetes, exogenous androgens, hypothyroidism, glucocorticoids, growth hormone, and advanced liver disease, and rises with estrogens, oral contraceptives, hyperthyroidism, anorexia, and ageing in men. Its main clinical use is correcting total testosterone interpretation by calculating free or bioavailable testosterone (Vermeulen formula). Inter-assay variability is large enough to alter clinical classification, so serial measurements should use the same platform.

Small dense LDL (sdLDL) is a subfraction of LDL particles characterised by reduced diameter (less than approximately 25.5 nm) and higher density compared with large buoyant LDL, arising primarily in states of hypertriglyceridaemia and insulin resistance through CETP-mediated triglyceride-for-cholesterol exchange. sdLDL particles are more atherogenic than large buoyant LDL for several reasons: they have lower affinity for the LDL receptor and therefore a longer plasma half-life, they penetrate the arterial intima more readily, they are more susceptible to oxidative modification, and they bind more avidly to proteoglycans in the subendothelial matrix. Elevated sdLDL concentration is independently associated with coronary artery disease and is a component of atherogenic dyslipidaemia — the lipid triad of elevated triglycerides, low HDL-C, and a predominance of sdLDL.

Testosterone is the principal androgen, produced mainly by Leydig cells in the testes in men and in smaller amounts by the ovaries and adrenals in women. It supports muscle mass, bone density, libido, mood, and erythropoiesis. Laboratories report total testosterone alongside free or bioavailable testosterone, since roughly 98% circulates bound to SHBG and albumin. Levels typically decline with age in men; lower values are observationally associated with sarcopenia, frailty, metabolic syndrome, and higher all-cause mortality and adverse cardiometabolic outcomes, with confounding by obesity, inflammation, and SHBG changes. The TRAVERSE trial (Lincoff et al. 2023 NEJM) found cardiovascular non-inferiority for testosterone therapy in hypogonadal men, with elevated atrial-fibrillation and pulmonary-embolism signals, informing the FDA's February 2025 class-wide label update that removed the boxed cardiovascular warning while retaining and strengthening warnings about increased blood pressure; atrial fibrillation and pulmonary embolism remain noted safety signals from TRAVERSE.

Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (TgAb) antibodies target intrathyroidal enzymes and are the principal serologic markers of autoimmune thyroid disease. They are measured by chemiluminescent or electrochemiluminescent immunoassay with assay-specific cutoffs (commonly anti-TPO <34 IU/mL and TgAb <115 IU/mL); results should always be interpreted with the manufacturer's reference range. Anti-TPO is positive in roughly 90% of Hashimoto's thyroiditis and approximately 75% of Graves' disease; TgAb is less specific but useful in differentiated thyroid cancer monitoring as it can interfere with thyroglobulin measurement. Per the 2017 ATA pregnancy guidelines, anti-TPO positivity increases the risk of miscarriage, preterm birth and postpartum thyroiditis, and influences treatment thresholds in subclinical hypothyroidism. Major confounders are assay heterogeneity, intercurrent viral infection, and other autoimmune conditions (type 1 diabetes, vitiligo) raising titres without thyroid dysfunction.

Transferrin saturation (TSAT) is the percentage of iron-binding sites on transferrin — the main iron-transport protein in blood — occupied by iron, calculated as (serum iron ÷ total iron-binding capacity) × 100. Physiologically, transferrin is 20–35% saturated, leaving reserve capacity to buffer free iron; the normal range is approximately 15–50% in adults, with values below 20% typical of iron deficiency. When TSAT exceeds 45–50%, non-transferrin-bound iron (NTBI) — a redox-active labile pool — appears in plasma and accumulates in hepatocytes, cardiomyocytes, and endocrine tissue, driving Fenton-reaction hydroxyl-radical generation, lipid peroxidation, and ferroptosis. Chronically elevated TSAT is associated with greater all-cause mortality. A NHANES I follow-up cohort (n = 10,714; Mainous et al., 2004) found HR 1.60 (95% CI 1.17–2.21) for TSAT > 55%; a NHANES II analysis by the same group showed elevated TSAT combined with high dietary iron roughly tripled mortality risk (HR 2.90; 95% CI 1.39–6.04). A fasting TSAT > 45% is the standard first-line screening threshold for hereditary hemochromatosis (HFE-related, most often C282Y homozygosity); EASL 2022 Clinical Practice Guidelines define iron overload as TSAT > 45% plus ferritin > 200 µg/L in women, or TSAT > 50% plus ferritin > 300 µg/L in men, with HFE genotyping indicated once either criterion is reproducibly exceeded. Because TSAT reflects iron flux rather than total stores, it complements ferritin and can detect preclinical iron excess before organ damage is measurable.

Triglycerides are the main storage form of dietary and endogenous fat, transported in the blood by triglyceride-rich lipoproteins, mainly VLDL, chylomicrons, IDL, and their remnants. Fasting triglyceride levels rise with insulin resistance, excess refined carbohydrates, alcohol intake, and metabolic syndrome. Elevated values reflect higher concentrations of triglyceride-rich remnant lipoproteins, and remnant cholesterol carried by these particles is causally linked to atherosclerotic cardiovascular disease (Mendelian randomization). Lower fasting triglycerides are associated with better insulin sensitivity and reduced cardiovascular risk.

Thyroid-stimulating hormone (TSH) is released by anterior pituitary thyrotrophs and regulates thyroid hormone production via negative feedback from circulating T3 and T4. In primary thyroid disease it is the most sensitive single marker of thyroid status: higher values typically indicate hypothyroidism, while suppressed values suggest hyperthyroidism or exogenous hormone excess. In central (pituitary or hypothalamic) hypothyroidism, TSH may be inappropriately normal or low alongside low fT4, so combined interpretation with free T4 is required. Reference ranges shift slightly upward with age, and subclinical dysfunction is common in older adults.

The TyG index is a surrogate marker for insulin resistance calculated as ln(fasting triglycerides [mg/dL] × fasting glucose [mg/dL] ÷ 2). Proposed by Simental-Mendía et al. in 2008, it approximates hyperinsulinemic-euglycemic clamp measurements without an insulin assay. The index reflects impaired triglyceride clearance and elevated hepatic glucose output, hallmarks of insulin resistance in adipose and liver tissue. Insulin resistance accelerates atherosclerosis, promotes visceral adiposity, and contributes to β-cell exhaustion; elevated TyG values are independently associated with biological aging phenotypes. A 2022 meta-analysis (12 cohorts, n ≈ 6.4 million) found the highest TyG quartile carried roughly doubled coronary artery disease risk (HR 2.01; 95% CI 1.68–2.40) and 46% higher composite CVD risk; mortality associations were weaker and heterogeneous. A 2023 Swedish cohort analysis (Malmö Diet and Cancer Study, Malmö Preventive Project) showed TyG increments predicted arterial stiffness, 3.3-fold diabetes incidence (top vs. bottom quartile), and higher all-cause (HR 1.22) and cardiovascular mortality (HR 1.37). All evidence is observational; TyG-guided interventions remain untested in randomized trials, and the index performs best in non-diabetic, insulin-naïve populations.

Uric acid is the final catabolic product of purine metabolism in humans, produced mainly in the liver by xanthine oxidase and excreted predominantly by the kidney (~70%) with the remainder through the gut. Hyperuricemia (typically defined as serum uric acid >6.8 mg/dL or >404 µmol/L — the saturation threshold above which monosodium urate crystals can form; some guidelines use sex-specific cut-offs of >6 mg/dL in women and >7 mg/dL in men, while <6 mg/dL / <360 µmol/L is the urate-lowering treat-to-target ceiling for established gout per ACR 2020 / EULAR 2016) is the prerequisite for monosodium urate crystal deposition, which causes gout arthritis and urate nephropathy. Beyond gout, persistently elevated uric acid is epidemiologically associated with hypertension, insulin resistance, metabolic syndrome, chronic kidney disease, and cardiovascular events; whether these associations are causal or confounded is debated, as Mendelian randomization results have been inconsistent. Uricosuria and renal tubular handling are strongly influenced by dietary purines (red meat, organ meat, beer, fructose) and medications (thiazides, low-dose aspirin, cyclosporine), and lower serum levels are associated with better metabolic health in most population studies.

The urine albumin-to-creatinine ratio (UACR) quantifies albumin leaking into urine relative to urinary creatinine, correcting for hydration status. A healthy glomerular filtration barrier retains virtually all albumin; persistent UACR ≥ 30 mg/g signals structural or functional breach of the glomerular basement membrane or endothelium, while values ≥ 300 mg/g define overt macroalbuminuria. Together with estimated GFR (eGFR), UACR forms one of the two pillars of the KDIGO 2024 CGA (Cause, GFR, Albuminuria) classification for chronic kidney disease staging and risk stratification. Beyond kidney health, UACR reflects systemic endothelial dysfunction: albumin escaping glomerular capillaries mirrors generalised vascular permeability, predicting cardiovascular events independently of traditional risk factors. In the PREVEND cohort (Hillege et al. 2002, n ≈ 40,500 general-population adults, Groningen), even sub-clinical albuminuria below 30 mg/g showed a continuous, graded association with all-cause and cardiovascular mortality, independent of blood pressure, diabetes status, and kidney function. A 2023 KNHANES-based study by Kim et al. (n = 19,340 Korean adults) confirmed that UACR above 30 mg/g is associated with elevated cardiovascular risk across diabetic and non-diabetic subgroups. Screening requires only a single first-morning or random mid-stream urine sample, but two or more elevated readings at least three months apart are needed to distinguish persistent from transient albuminuria.

Vitamin B12 (cobalamin) and folate (vitamin B9) are essential coenzymes in one-carbon metabolism: B12 is required for methionine synthase (which regenerates methionine from homocysteine) and methylmalonyl-CoA mutase, while folate supplies methyl groups via the folate cycle. Deficiency of either causes megaloblastic anemia by impairing DNA synthesis; B12 deficiency additionally produces neurological damage — including subacute combined degeneration of the spinal cord — through mechanisms not fully replicated by folate. Low B12 is prevalent in older adults, strict vegans, and those taking metformin or proton pump inhibitors. Functional B12 status is more accurately reflected by elevated methylmalonic acid and holotranscobalamin (active-B12) than by total serum B12 alone, which can be misleadingly normal. Both markers are clinically used to investigate elevated homocysteine and associated cardiovascular and cognitive risk.

25-hydroxyvitamin D (25-OH-D, calcidiol) is the major circulating form of vitamin D and the standard clinical biomarker for assessing vitamin D status, produced in the liver by hydroxylation of cholecalciferol (D3) or ergocalciferol (D2). It reflects both dietary intake and skin synthesis from UVB exposure; its half-life of roughly 2–3 weeks makes it a reliable index of longer-term status compared with the active hormonal form 1,25-(OH)₂D. Deficiency (commonly defined as <50 nmol/L or <20 ng/mL) is associated with impaired bone mineralization, falls, increased susceptibility to respiratory infection, and observationally with higher all-cause and cardiovascular mortality, although randomized trials have shown mixed results for supplementation on hard outcomes. The 2024 Endocrine Society clinical practice guideline (Demay et al., JCEM) moved away from threshold-based deficiency definitions for asymptomatic adults — finding insufficient clinical-trial evidence to tie specific 25(OH)D cutoffs to outcomes — and recommended empiric supplementation for selected groups (<18 years, >75 years, pregnancy, high-risk prediabetes) rather than universal screening. Many longevity-oriented practitioners still target 75–100 nmol/L despite the guideline shift.