Plain-language explainers covering 73 common blood test markers. Each page explains what the test measures, normal ranges, what high or low results mean, and, most importantly, which other markers should be read alongside it for full clinical context.
ALT is an enzyme found primarily in liver cells. When liver cells are damaged or inflamed, ALT leaks into the bloodstream, making it one of the most sensitive markers for liver inj…
LiverAlbumin is the most abundant protein in blood, produced exclusively by the liver. It maintains fluid balance in blood vessels, transports hormones and drugs, and serves as a key ma…
LiverALP is an enzyme concentrated in the liver (bile duct cells), bone (osteoblasts), intestine, and placenta. Elevated ALP most often points to either liver/biliary disease or bone di…
LiverAST is an enzyme found in liver, heart, skeletal muscle, kidneys, brain, and red blood cells. Because it is present in multiple tissues, AST is less liver-specific than ALT. The AS…
LiverGGT is an enzyme found primarily in the liver and bile ducts. It is the most sensitive routinely available marker for bile duct injury and is also induced by alcohol and certain me…
LiverBilirubin is a yellow-orange pigment from the normal breakdown of hemoglobin in aged red blood cells. The liver processes (conjugates) bilirubin for excretion in bile. Elevated bil…
LiverTotal protein measures all proteins in blood, primarily albumin (~60%) and globulins (~40%). Globulins include antibodies, transport proteins, and clotting factors. The albumin-to…
BUN measures urea nitrogen, a waste product from protein breakdown in the liver. Unlike creatinine, BUN is affected by dietary protein, hydration, GI bleeding, and liver function, …
KidneyCreatinine is a waste product from normal muscle metabolism, produced at a relatively constant rate. It is freely filtered by the kidneys with minimal reabsorption, making it the m…
KidneyCystatin C is a small protein produced at a constant rate by all nucleated cells. It is freely filtered by the kidneys. Unlike creatinine, it is not affected by muscle mass, age, s…
KidneyeGFR estimates how much blood the kidneys filter per minute, calculated from serum creatinine, age, and sex using the CKD-EPI equation. It is the primary metric for detecting, stag…
KidneyUric acid is the end product of purine metabolism. Purines come from normal cell turnover and diet (red meat, organ meats, seafood, beer, fructose). About two-thirds is excreted by…
ApoB is the primary protein of all atherogenic particles, LDL, VLDL, IDL, and Lp(a). Each particle carries exactly one ApoB molecule, so ApoB directly counts plaque-promoting part…
LipidsHigh-density lipoprotein (HDL), " good cholesterol" , removes excess cholesterol from artery walls and transports it back to the liver for excretion (reverse cholesterol transport)…
LipidsLow-density lipoprotein (LDL) is the primary carrier of cholesterol to tissues. Excess LDL particles enter the arterial wall, become oxidized, and drive the atherosclerotic plaque …
LipidsLp(a) is an LDL-like particle with an extra protein, apolipoprotein(a). It is ~90% genetically determined; diet and exercise have little impact. Elevated Lp(a) is an independent, …
LipidsNon-HDL cholesterol = total cholesterol minus HDL. It represents all atherogenic (plaque-promoting) particles: LDL, VLDL, IDL, and Lp(a). Many lipid experts consider it a better ri…
LipidsTotal cholesterol is the sum of LDL, HDL, and VLDL cholesterol. It is a screening tool for cardiovascular risk, though the individual components, especially LDL and HDL, are far …
LipidsTriglycerides are the most common type of fat in the body and the primary form of energy storage. After eating, excess calories are converted to triglycerides. Blood levels are the…
Eosinophils (1–4% of WBCs) defend against parasites and mediate allergic responses. Their granules contain proteins toxic to parasites. They play central roles in asthma, atopic di…
CBCHematocrit is the percentage of blood volume occupied by red blood cells. It is roughly 3× the hemoglobin (the " rule of three"). Hematocrit is affected by plasma volume, dehydrati…
CBCHemoglobin is the iron-containing protein inside red blood cells that binds and transports oxygen. It is the most clinically important CBC parameter, hemoglobin concentration defi…
CBCLymphocytes (20–40% of WBCs) are the cornerstone of adaptive immunity. They include T cells (cell-mediated immunity), B cells (antibody production), and NK cells (innate defense ag…
CBCMCH is the average amount of hemoglobin per red blood cell. It generally tracks with MCV, smaller cells hold less hemoglobin. Low MCH indicates hypochromia (pale cells from insuff…
CBCMCHC is the average hemoglobin concentration within RBCs. Unlike MCH, it corrects for cell size. MCHC is most useful for identifying hereditary spherocytosis and detecting lab arti…
CBCMCV measures the average size of red blood cells. It is the single most useful parameter for classifying anemias: microcytic (<80 fL, small cells), normocytic (80–100 fL), or macro…
CBCMonocytes are the largest white blood cells (2–10% of WBCs). They circulate briefly before migrating into tissues and becoming macrophages and dendritic cells, the body's professi…
CBCNeutrophils are the most abundant white blood cells (50–70% of WBCs). They are the first line of defense against bacterial and fungal infections. Short-lived (hours to ~1 day), the…
CBCPlatelets (thrombocytes) are small cell fragments essential for blood clotting. They adhere to injury sites, aggregate, and activate the clotting cascade. Platelet count evaluates …
CBCRed blood cells (erythrocytes) carry oxygen from lungs to tissues via hemoglobin. RBCs survive ~120 days. The RBC count works with hemoglobin and hematocrit to evaluate oxygen-carr…
CBCRDW measures the variation in RBC size (anisocytosis). High RDW means RBCs vary widely. It is the earliest CBC sign of iron deficiency (rising before MCV drops) and a key discrimin…
CBCWhite blood cells (leukocytes) are the immune system's circulating defense cells. The total WBC count sums all types; the differential breaks it into neutrophils, lymphocytes, mono…
Ferritin is the primary iron storage protein. Serum ferritin reflects total body iron stores and is the single best test for diagnosing iron deficiency. Ferritin is also an acute p…
IronSerum iron measures the amount of iron circulating in the blood, mostly bound to transferrin. Serum iron fluctuates significantly; it is higher in the morning, after iron-rich mea…
IronTIBC measures the blood's total capacity to bind iron; it is an indirect measure of transferrin, the primary iron transport protein. TIBC rises when the body needs more iron (iron…
IronTransferrin saturation (TSAT) is the percentage of transferrin binding sites occupied by iron. It is calculated as: TSAT (%) = (serum iron ÷ TIBC) × 100. TSAT is a key parameter fo…
T3 (triiodothyronine) is the active thyroid hormone, about 4× more potent than T4. Most T3 (~80%) is produced by peripheral conversion of T4 (deiodination in liver, kidney, and ot…
ThyroidT4 (thyroxine) is the primary hormone produced by the thyroid gland. Most T4 in blood is protein-bound; free T4 is the unbound, biologically available fraction (~0.03% of total T4)…
ThyroidTSH is produced by the pituitary gland and stimulates the thyroid to produce T4 and T3. TSH is the single best screening test for thyroid function; it rises when thyroid hormones …
ThyroidThyroid peroxidase (TPO) antibodies are autoantibodies directed against thyroid peroxidase, an enzyme critical for thyroid hormone synthesis. Elevated TPO antibodies are the hallma…
C-peptide is a protein fragment cleaved from proinsulin when insulin is produced. It is released in equimolar amounts with insulin but is not extracted by the liver (unlike insulin…
DiabetesFasting blood glucose measures the amount of sugar in the blood after an 8–12 hour fast. It is the most common screening test for diabetes and prediabetes. Glucose is the body's pr…
DiabetesHbA1c measures the percentage of hemoglobin with glucose attached (glycated hemoglobin). Because red blood cells survive ~120 days, HbA1c reflects average blood glucose over the pr…
DiabetesInsulin is a hormone produced by pancreatic beta cells that enables cells to take up glucose from the blood. Fasting insulin, together with fasting glucose, is used to assess insul…
Calcium is essential for bone structure, nerve signaling, muscle contraction, blood clotting, and enzyme function. About 40% of serum calcium is protein-bound (primarily to albumin…
ElectrolytesChloride is the primary extracellular anion, accompanying sodium to maintain electrical neutrality and osmotic balance. Chloride is passively absorbed and excreted, largely trackin…
ElectrolytesMagnesium is the fourth most abundant cation in the body, essential for over 300 enzymatic reactions including ATP utilization, DNA/protein synthesis, nerve conduction, and muscle …
ElectrolytesPhosphorus is essential for bone mineralization (as hydroxyapatite with calcium), energy storage (ATP), cell membranes (phospholipids), and acid-base buffering. It is primarily int…
ElectrolytesPotassium is the primary intracellular cation, essential for nerve conduction, muscle contraction (including the heart), and maintaining cell membrane potential. Serum potassium is…
ElectrolytesSodium is the primary extracellular cation and the main determinant of plasma osmolality and fluid balance. Sodium concentration reflects water balance more than sodium balance, l…
Folate is a water-soluble B vitamin essential for DNA synthesis, cell division, and red blood cell formation. It works closely with vitamin B12 in one-carbon metabolism. Adequate f…
VitaminsVitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, red blood cell formation, and neurological function (myelin sheath maintenance). It is found only in…
VitaminsVitamin D is a fat-soluble vitamin and hormone precursor essential for calcium absorption, bone mineralization, immune function, and muscle strength. 25-hydroxyvitamin D (25(OH)D) …
C-reactive protein is an acute phase reactant produced by the liver in response to inflammation. It rises within hours of an inflammatory trigger and falls rapidly once resolved. C…
InflammationESR measures how quickly red blood cells settle in a tube of anticoagulated blood over one hour. Inflammation causes RBCs to aggregate (rouleaux formation) and settle faster. ESR i…
InflammationFibrinogen is a soluble plasma protein (clotting factor I) produced by the liver. It is the final step in the coagulation cascade, thrombin converts fibrinogen to insoluble fibrin…
InflammationHigh-sensitivity CRP (hs-CRP) is a more sensitive version of the standard CRP test, capable of measuring very low levels of inflammation that the standard CRP test cannot detect. I…
InflammationHomocysteine is an amino acid produced during methionine metabolism. It is normally rapidly converted to cysteine (with vitamin B6) or remethylated to methionine (with folate and B…
BNP (B-type natriuretic peptide) and NT-proBNP (N-terminal proBNP) are hormones released by heart muscle cells (cardiomyocytes) in response to stretching, specifically from increa…
CardiacCreatine kinase is an enzyme found in skeletal muscle, heart muscle, and brain. It catalyzes the conversion of creatine to phosphocreatine, consuming ATP, a key reaction in energy…
CardiacTroponin is a protein complex found in heart muscle cells that regulates contraction. When heart muscle is injured, troponin leaks into the blood. High-sensitivity cardiac troponin…
Cortisol is the primary glucocorticoid hormone, produced by the adrenal cortex under ACTH stimulation from the pituitary. It follows a strong circadian rhythm, highest in the earl…
HormonesDHEA-S is an androgen precursor produced primarily by the adrenal glands. It is the most abundant circulating steroid hormone. DHEA-S levels decline steadily with age (adrenopause)…
HormonesProlactin is a hormone produced by the anterior pituitary gland. Its primary function is lactation (milk production) after childbirth. Prolactin is normally elevated during pregnan…
HormonesSHBG is a liver-produced protein that binds testosterone and estradiol in the blood. Only unbound (free) hormone is biologically active. SHBG levels determine how much total testos…
HormonesTestosterone is the primary male sex hormone (androgen), produced primarily by the testes in men and in smaller amounts by the ovaries and adrenal glands in women. It regulates lib…
D-dimer is a fibrin degradation product; it is produced when a blood clot is broken down by the fibrinolytic system. D-dimer is the most sensitive biomarker for excluding venous t…
CoagulationPTT (also called aPTT, activated partial thromboplastin time) measures the intrinsic and common coagulation pathways (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). It is u…
CoagulationProthrombin time (PT) measures the extrinsic and common coagulation pathways (factors VII, X, V, II, and fibrinogen). It is used to monitor warfarin therapy (reported as INR, Inte…
Ammonia is a neurotoxic waste product of protein metabolism. The liver converts ammonia to urea (via the urea cycle) for excretion by the kidneys. Elevated ammonia primarily indica…
OtherLDH is an enzyme found in nearly every cell in the body. It is a non-specific marker of cellular injury and turnover. LDH is elevated in a wide variety of conditions; its value li…
OtherLipase is an enzyme produced by the pancreas that breaks down dietary fats. It is the preferred test for acute pancreatitis, more sensitive and specific than amylase. Lipase rises…
These explainers are for educational purposes. BloodWorker reads your markers together, not one at a time, to find the patterns a thoughtful clinician would notice. Upload your bloodwork to see your full picture.
