Every Lab Biomarker, Explained
61 blood test biomarkers in plain English. Reference ranges, what high and low results mean, when to act, and how each value connects to the others on your panel. Reviewed by clinicians. No paywall.
All biomarkers (A–Z)
Complete Blood Count
(9)Hemoglobin Levels: Normal Range, Low & High Results Explained
Understand your hemoglobin blood test results. See normal hemoglobin levels by age and sex, what low or high hemoglobin means, and when to talk to your doctor.
Read guideComplete Blood Count (CBC): What Each Value Means and How to Read the Whole Panel
The CBC is the most common blood test in medicine. What hemoglobin, hematocrit, RBC, MCV, MCH, RDW, white blood cells, and platelets each mean — and how to read them as a single picture, not 25 isolated numbers.
Read guideHematocrit (HCT): Normal Range, What High and Low Mean
Hematocrit is the percentage of your blood made up of red blood cells. Normal range, what high and low values mean, why it tracks tightly with hemoglobin, and how dehydration distorts it.
Read guideRed Blood Cell (RBC) Count: Normal Range, What High and Low Mean
The RBC count is the number of red blood cells per microliter. Normal range, what high and low values mean, and why RBC alone is rarely the most useful number on a CBC.
Read guideMCV (Mean Corpuscular Volume): Normal Range, What High and Low Mean
MCV is the average size of your red blood cells. Normal range is 80–100 fL. Low MCV (microcytic) usually means iron deficiency. High MCV (macrocytic) usually means B12, folate, alcohol, or hypothyroidism.
Read guideMCH (Mean Corpuscular Hemoglobin): Normal Range, What Low and High Mean
MCH is the average amount of hemoglobin per red blood cell. Normal range is 27–33 pg. Low MCH (hypochromic) usually means iron deficiency or thalassemia. What MCH adds beyond MCV.
Read guideRDW (Red Cell Distribution Width): The Underrated CBC Number
RDW measures variation in red blood cell size. Normal range is 11.5–14.5%. High RDW is one of the earliest signs of nutritional deficiency and a quietly powerful predictor of cardiovascular and all-cause mortality.
Read guideWhite Blood Cell (WBC) Count: How to Read the Differential
WBC count alone tells you very little, the differential drives interpretation. What high and low WBC mean, the patterns of bacterial vs viral vs allergic vs malignant disease, and when neutropenia is an emergency.
Read guidePlatelets: How to Read Your Platelet Count
Platelets clot blood. What thrombocytopenia (low) and thrombocytosis (high) mean, why iron deficiency raises platelets, why EDTA clumping causes false-low results, and when platelet counts are a bleeding emergency.
Read guideDiabetes & Glucose
(4)HbA1c Normal Range, A1c Levels Chart & What Your Results Mean
Learn what HbA1c levels mean, see the A1c chart for normal vs. prediabetes vs. diabetes ranges, and find out how to convert A1c to average blood sugar.
Read guideFasting Glucose: Normal Range, What High and Low Results Mean
Fasting blood sugar normal range is below 100 mg/dL. See the fasting glucose chart, what 100–125 and 126+ mean, and why a single high reading often is not prediabetes.
Read guideBlood Glucose: Normal Range, Levels Chart, and What Your Number Means
Blood sugar levels chart for fasting, post-meal, and random glucose. See the normal glucose range, when a reading is prediabetes or diabetes, and how to read a single number.
Read guideFasting Insulin and HOMA-IR: The Earliest Sign of Insulin Resistance
Fasting insulin detects insulin resistance years before HbA1c rises. Optimal under 7 µIU/mL, HOMA-IR over 2.5 suggests resistance. When to order, what high values mean, and how the number falls with low-carb intervention.
Read guideLipid Panel
(6)Total Cholesterol: What Your Numbers Actually Mean [2026 Guide]
Total cholesterol levels explained with 2026 ACC/AHA guidelines. Normal ranges, chart by age, the ratio debate, and why trends matter more than one reading.
Read guideLDL Cholesterol: Normal Range, "Bad Cholesterol" Targets Explained
LDL cholesterol normal range, age-based targets, what high LDL really means, and why the 2026 ACC/AHA guidelines lowered the threshold. Practical, opinionated answers.
Read guideHDL Cholesterol: Normal Range and Why "Higher Is Better" Has Limits
HDL cholesterol normal range, what high and low HDL mean, the U-shape risk curve, and why simply raising HDL with medication does not work. 2026 evidence-based guide.
Read guideTriglycerides: Normal Range, What High Levels Mean, How to Lower Them
Triglycerides normal range is below 150 mg/dL. What 150–500 means, why 500+ is a pancreatitis emergency, and the trig/HDL ratio nobody talks about. 2026 evidence-based guide.
Read guideApolipoprotein B (ApoB): The Better Cholesterol Number Most People Have Never Heard Of
Apolipoprotein B (ApoB) measures the actual number of atherogenic particles. The 2026 ACC/AHA guidelines recommend it for diabetes, metabolic syndrome, and known CVD. Targets and how to lower ApoB.
Read guideLipoprotein(a) [Lp(a)]: The Genetic Heart Disease Risk Factor Everyone Should Test Once
Lipoprotein(a) is a genetically determined cardiovascular risk factor unaffected by diet or exercise. The 2026 ACC/AHA guidelines now recommend testing every adult once. Targets, what the result means, and what you can actually do about it.
Read guideLiver Function
(6)ALT (Alanine Aminotransferase): What Your Liver Enzyme Says About Your Health
ALT is the most sensitive blood test for liver injury. Normal range, what mildly elevated ALT means (almost always fatty liver), when to worry, and the new MASLD/NAFLD diagnostic landscape in 2026.
Read guideAST (Aspartate Aminotransferase): Normal Range, What High AST Means
AST is a liver and muscle enzyme paired with ALT on every metabolic panel. Normal range, what high AST means, the AST/ALT ratio that distinguishes alcohol from fatty liver, and when to act.
Read guideAlkaline Phosphatase (ALP): Normal Range, What High ALP Means
Alkaline phosphatase rises in liver, bile duct, and bone disease — and the patterns differ. Normal range, how GGT distinguishes liver from bone, why pediatric values are far higher, and when ALP warrants imaging.
Read guideBilirubin (Total, Direct, Indirect): Normal Range, What High Bilirubin Means
Total bilirubin is the sum of direct and indirect — and the split is what matters. Normal range, why high indirect usually means hemolysis or Gilbert's syndrome, why high direct points at biliary obstruction, and when jaundice warrants urgent care.
Read guideAlbumin: Normal Range, What Low Albumin Means
Albumin reflects liver synthetic function, nutritional status, and chronic illness — and it is the value used to correct calcium. Normal range, why low albumin matters more than high, and when to act.
Read guideGGT (Gamma-Glutamyl Transferase): Normal Range, What High GGT Means
GGT is the most sensitive blood marker for biliary disease and alcohol use. Normal range, why it sorts liver from bone when ALP is high, what raises it (alcohol, NAFLD, medications), and when it warrants action.
Read guideKidney Function
(2)Creatinine and eGFR: How to Read Your Kidney Function Test
Creatinine reflects kidney filtration; eGFR translates it into a useful number. Normal ranges, what high creatinine means, why a single elevated reading does not equal kidney disease, and the 2026 race-free eGFR equation.
Read guideBUN (Blood Urea Nitrogen): How to Read Your Kidney and Hydration Marker
BUN reflects both kidney clearance and hydration. What high and low BUN mean, how the BUN/creatinine ratio distinguishes dehydration from intrinsic kidney disease, and why protein intake and GI bleeding shift the result.
Read guideMetabolic Panel
(1)Metabolic
(1)Electrolytes
(4)Sodium: How to Read Your Blood Sodium Level
Sodium is the dominant electrolyte in blood and the master controller of fluid balance. Normal range, what high and low sodium mean, why most hyponatremia is dilutional, and when a sodium value is genuinely dangerous.
Read guidePotassium: How to Read Your Blood Potassium Level
Potassium is tightly regulated because heart rhythm depends on it. Normal range, what high and low potassium mean, why a slightly elevated K+ is usually a hemolyzed sample, and when potassium values become an arrhythmia risk.
Read guideMagnesium: The Underdiagnosed Electrolyte Deficiency
Magnesium deficiency is common, often missed on routine panels, and the hidden cause of refractory hypokalemia and hypocalcemia. Normal range, what low magnesium means, why PPIs and diuretics deplete it, and when to supplement.
Read guideCalcium: How to Read Your Blood Calcium Level
Blood calcium is tightly regulated by parathyroid hormone and vitamin D. Normal range, what high and low calcium mean, why you must correct for albumin, and the most common outpatient cause of hypercalcemia.
Read guideThyroid
(3)TSH (Thyroid-Stimulating Hormone): Normal Range, What High and Low Mean
TSH normal range is roughly 0.4–4.0 mIU/L, but the optimal range is narrower than most people realize. What high and low TSH mean, why a single test is rarely enough, and when to retest.
Read guideFree T4 (Thyroxine): Normal Range, What High and Low Mean
Free T4 normal range is roughly 0.8–1.8 ng/dL. The right second test after TSH — what high and low Free T4 mean, how the TSH–T4 pattern points to hypothyroidism vs hyperthyroidism vs pituitary disease.
Read guideFree T3 (Triiodothyronine): Normal Range, When It Matters Most
Free T3 normal range is roughly 2.3–4.2 pg/mL. Why it is the right test when hyperthyroidism is suspected, when to order it on top of TSH and Free T4, and what high and low results actually mean.
Read guideVitamins & Nutrients
(3)Vitamin D (25-Hydroxy): Normal Range, Deficiency, How Much to Take
Vitamin D normal range is 30–100 ng/mL, but the optimal target is debated. What deficiency means, how much D3 to supplement, and why a single low reading often reverses with sunshine and a few capsules.
Read guideVitamin B12: Normal Range, Deficiency Symptoms, Why "Normal" Is Often Too Low
Vitamin B12 normal range is roughly 200–900 pg/mL, but the lab cutoff misses meaningful deficiency. What low B12 actually causes, why methylmalonic acid is the better test, and how to supplement.
Read guideFolate (Vitamin B9): Normal Range, Deficiency, Why It Must Be Checked With B12
Folate normal range is above ~4 ng/mL. Why folate deficiency causes the same anemia as B12 deficiency, why supplementing folate alone can mask B12 deficiency, and when low folate matters most.
Read guideIron Studies
(2)Ferritin: Normal Range, What Low and High Levels Mean for Iron Stores
Ferritin is the best single marker of body iron stores. Normal range, why low ferritin causes fatigue before anemia, and what high ferritin means (inflammation, hemochromatosis, fatty liver).
Read guideIron Studies (Serum Iron, TIBC, Transferrin Saturation): How to Read Them Together
Serum iron, TIBC, and transferrin saturation are the three numbers that — together with ferritin — diagnose iron deficiency, iron overload, and the anemia of chronic disease.
Read guideInflammation
(4)High-Sensitivity C-Reactive Protein (hs-CRP): What Inflammation Says About Your Heart Risk
hs-CRP measures low-level inflammation linked to cardiovascular risk. Normal range, what high hs-CRP means, why a single elevated reading is rarely meaningful, and 2026 evidence-based guidance.
Read guideErythrocyte Sedimentation Rate (ESR): What This Old Inflammation Marker Still Tells You
ESR (sed rate) is a slow, nonspecific inflammation marker. Normal ranges by age and sex, what high ESR (and ESR over 100) means, when ESR beats CRP, and why one elevated reading rarely settles anything.
Read guideC-Reactive Protein (CRP): What This Acute Inflammation Marker Means
Standard CRP detects clinically meaningful inflammation and infection. Normal range, what high CRP means, why it is not the same test as hs-CRP, and how it tracks recovery within days.
Read guideFibrinogen: A Clotting Factor and Inflammation Marker in One
Fibrinogen is both a clotting factor and an acute-phase reactant. Normal range 200–400 mg/dL, what high fibrinogen means for clot risk, what low fibrinogen means for bleeding, and why ESR moves with it.
Read guideCardiovascular markers
(1)Hormones
(10)Total Testosterone: Normal Range, What High and Low Mean (Men and Women)
Total testosterone normal range for adult men is roughly 264–916 ng/dL, but levels swing with time of day, age, and illness. What low T really means, when treatment is justified, and why a single number is rarely enough.
Read guideFree Testosterone: Normal Range, Why It Matters When SHBG Is Off
Free testosterone is the bioavailable fraction (~2% of total) and the test that disambiguates real androgen status when SHBG is high or low. Normal ranges by sex and age, calculated vs. measured, when to order it.
Read guideSHBG (Sex Hormone-Binding Globulin): What High and Low Mean
SHBG is the carrier protein that decides how much of your testosterone and estradiol is biologically available. Low SHBG flags insulin resistance and metabolic disease; high SHBG flags hyperthyroidism, oral estrogen, or liver disease.
Read guideDHEA-S: Normal Range by Age, What High and Low Mean
DHEA-S is the most abundant adrenal androgen and a useful marker of adrenal androgen production. Normal range by age, when high DHEA-S points to PCOS or adrenal disease, and the (limited) evidence for DHEA supplementation.
Read guideEstradiol (E2): Normal Range by Cycle Phase, Menopause, and in Men
Estradiol is the primary estrogen and the most clinically useful one to measure. Reference ranges by menstrual cycle phase, postmenopausal cutoffs, the small but real range in men, and how estradiol guides fertility and HRT decisions.
Read guideProgesterone: Normal Range by Cycle Phase, Pregnancy, and Day-21 Testing
Progesterone confirms ovulation and supports early pregnancy. Mid-luteal (day 21) progesterone above 5 ng/mL confirms ovulation. Reference ranges by cycle phase and trimester, and what high or low progesterone means.
Read guideFSH (Follicle-Stimulating Hormone): Normal Range, Menopause, Day-3 Testing
FSH drives follicle development in women and sperm production in men. Day 3 FSH evaluates ovarian reserve; sustained FSH above 25 mIU/mL with amenorrhea confirms menopause. Reference ranges by cycle phase and what high or low FSH means.
Read guideLH (Luteinizing Hormone): Normal Range, the Ovulation Surge, LH/FSH Ratio
LH triggers ovulation and stimulates testosterone production in men. Normal range by cycle phase, the mid-cycle surge, the LH/FSH ratio in PCOS, and what high or low LH means.
Read guideProlactin: Normal Range, Causes of High Prolactin, and When to Image the Pituitary
Prolactin above 25 ng/mL in women or 20 in men is hyperprolactinemia. Causes range from medications and hypothyroidism to prolactinomas, values above 100–150 ng/mL strongly suggest a pituitary adenoma. What to test, what to image, and when to treat.
Read guideAMH (Anti-Müllerian Hormone): Ovarian Reserve, IVF Planning, PCOS
AMH is the best single marker of ovarian reserve and a key planning number for fertility, IVF response, and timing decisions. Reference ranges by age, what high AMH means in PCOS, and what low AMH says about menopause timing.
Read guideTumor Markers
(5)PSA (Prostate-Specific Antigen): What the Test Actually Tells You About Prostate Cancer Risk
PSA is a prostate health marker, not a cancer test. Normal ranges, why a single elevated value rarely means cancer, age-specific cutoffs, free-PSA ratio, and the 2026 evidence on screening vs. monitoring.
Read guideCA-125: A Useful Ovarian Cancer Monitoring Tool, Not a Screening Test
CA-125 is widely misunderstood. It is not recommended for ovarian cancer screening in average-risk women. Normal range, common benign causes of elevation, and where the test is genuinely evidence-based in 2026.
Read guideCA 19-9: A Pancreatic Cancer Monitoring Tool with Big Caveats
CA 19-9 is not a pancreatic cancer screening test. About 5–10% of people genetically cannot make it. Where CA 19-9 is genuinely useful, why so many elevations are benign, and the 2026 evidence on monitoring known disease.
Read guideCEA (Carcinoembryonic Antigen): Colorectal Cancer Surveillance, Not Screening
CEA is a colorectal cancer monitoring tool, not a screening test. Smokers run higher baselines. Where CEA is genuinely useful (surveillance after surgery) and why elevations in healthy people are rarely cancer.
Read guideAFP (Alpha-Fetoprotein): Liver Cancer Surveillance, Germ Cell Tumors, and Pregnancy Screening
AFP is the main blood-based marker for hepatocellular carcinoma surveillance in cirrhosis, and a tumor marker for testicular and ovarian germ cell tumors. Normal range, why it is not a general cancer screen, and how to interpret elevations.
Read guideTrack your lab results over time
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