How to Read Blood Test Results: A Plain-Language Guide to Every Major Biomarker

Getting your blood work back and seeing a wall of numbers, abbreviations, and reference ranges is confusing — and most physicians spend less than 5 minutes reviewing results with patients. This guide translates the most important blood biomarkers from medical jargon into plain language, explains what "in range" actually means vs. what's optimal, and tells you what to do when something is flagged.

The Key Distinction: Reference Range vs Optimal Range

Every lab report includes a "reference range" — typically the middle 95% of the population tested at that lab. These ranges are population statistics, not health standards. Someone in the middle of the reference range can still be metabolically dysfunctional; someone slightly outside can be perfectly healthy.

Optimal ranges are derived from longitudinal studies correlating biomarker levels with health outcomes — not from population distribution. For many markers, the optimal range is narrower and positioned differently than the reference range. This guide includes both.

Complete Blood Count (CBC)

Hemoglobin and Hematocrit

What it measures: oxygen-carrying capacity of blood. Reference range: Hgb 13.5-17.5 g/dL (men), 12-15.5 g/dL (women) Optimal: Upper half of reference range for your sex and age. Low (anemia): Fatigue, poor exercise tolerance. Causes: iron deficiency (most common), B12/folate deficiency, chronic disease. High (erythrocytosis): Can indicate dehydration or, if persistent, polycythemia vera (rare). Elite endurance athletes typically run high-normal.

WBC (White Blood Cell Count)

What it measures: immune cell count. Reference range: 4.5-11.0 × 10³/μL Low: May indicate viral suppression, certain medications, or bone marrow issues. High (leukocytosis): Bacterial infection, inflammation, leukemia (if very high with abnormal differential). Optimal: 4.5-7.5 × 10³/μL — upper end of normal without infection suggests chronic inflammation.

Platelets

Reference range: 150-400 × 10³/μL Low (thrombocytopenia): Bleeding risk; causes include certain medications, ITP, liver disease. High (thrombocytosis): Clotting risk; can be reactive (inflammation, iron deficiency) or primary (myeloproliferative disorder).

Comprehensive Metabolic Panel (CMP)

Glucose (Fasting)

Reference range: 70-99 mg/dL Optimal range: 72-85 mg/dL Prediabetes: 100-125 mg/dL (impaired fasting glucose) Diabetes: ≥126 mg/dL on two occasions Action: Fasting glucose 90-99 mg/dL warrants tracking and investigation — it's "normal" by reference but associated with increasing insulin resistance.

HbA1c (Glycated Hemoglobin)

What it measures: average glucose over 3 months. Reference range: <5.7% (normal) Optimal: 4.8-5.2% Prediabetes: 5.7-6.4% | Diabetes: ≥6.5% Why it matters: Even HbA1c of 5.3-5.6% (well within "normal") is associated with meaningful cardiovascular risk increase in large cohort studies.

Creatinine and eGFR (Kidney Function)

Creatinine reference range: 0.7-1.3 mg/dL (men), 0.6-1.1 mg/dL (women) eGFR (estimated glomerular filtration rate): >90 is normal; 60-89 mildly reduced; <60 indicates CKD stage 3. Note: Athletes and people taking creatine supplements run higher creatinine that's not pathological — always disclose supplement use to your physician.

ALT and AST (Liver Enzymes)

Reference range: ALT 7-56 U/L | AST 10-40 U/L Optimal: ALT <25 U/L (men), <19 U/L (women) based on recent NALFD research Elevated: Liver inflammation from NAFLD, alcohol, medications (statins, NSAIDs), viral hepatitis. Mild elevation (1-3x reference): Common and usually benign; retest in 6-12 weeks.

BMP Electrolytes (Sodium, Potassium, CO2, BUN)

• Sodium: 136-145 mEq/L — dysregulation indicates fluid/kidney issues
• Potassium: 3.5-5.0 mEq/L — critical for cardiac rhythm; <3.5 warrants attention
• CO2 (bicarbonate): 23-29 mEq/L — reflects acid-base balance
• BUN (blood urea nitrogen): 7-20 mg/dL — high with kidney disease or dehydration; low with liver disease or very low protein intake

Lipid Panel

Total Cholesterol

Reference range: <200 mg/dL desirable Critical nuance: Total cholesterol is a poor standalone predictor of cardiovascular risk. High total cholesterol from high HDL is not the same risk profile as high total cholesterol from high LDL and low HDL.

LDL Cholesterol

Reference range: <100 mg/dL optimal, <130 acceptable What matters more: LDL particle number (LDL-P) and small dense LDL — these are more predictive than LDL-C (cholesterol concentration). Standard lipid panels don't include LDL-P; request an NMR LipoProfile for more complete assessment. Statin thresholds: Based on 10-year cardiovascular risk calculation, not just LDL level.

HDL Cholesterol

Reference range: >40 mg/dL (men), >50 mg/dL (women) Optimal: >60 mg/dL HDL is protective — every 1 mg/dL increase in HDL is associated with 2-3% reduction in cardiovascular risk.

Triglycerides

Reference range: <150 mg/dL Optimal: <100 mg/dL High triglycerides (>200 mg/dL) with low HDL is the metabolic syndrome lipid pattern — strongly associated with insulin resistance. More responsive to dietary carbohydrate than dietary fat.

ApoB (Apolipoprotein B)

This is not on standard lipid panels but is the most accurate single marker for atherogenic particle count. Each atherogenic particle (LDL, VLDL, IDL) carries one ApoB molecule. Optimal: <80 mg/dL for general population, <70 mg/dL for high-risk individuals. Ask your physician to add ApoB to your lipid testing — it's inexpensive and superior to standard LDL-C.

Thyroid Panel

TSH (Thyroid Stimulating Hormone)

Reference range: 0.4-4.0 mIU/L Optimal: 0.5-2.5 mIU/L TSH is the best screening test — elevated TSH means the pituitary is working hard to stimulate a sluggish thyroid; low TSH means the pituitary is backing off because thyroid is overactive.

Free T3 and Free T4

If TSH is abnormal, these refine the diagnosis. T4 is the storage form; T3 is the active metabolite. Some individuals convert T4 to T3 poorly despite normal TSH — this is a clinical gray area not captured by TSH alone.

Key Hormones

Fasting Insulin

Reference range: 2-25 mIU/L Optimal: <5-7 mIU/L **This is not on standard metabolic panels — request it separately.** Fasting insulin >10 mIU/L with normal fasting glucose is a hallmark of insulin resistance (elevated insulin maintaining normal glucose, the early stage of metabolic dysfunction). Extremely sensitive marker.

Testosterone (Men)

Total testosterone reference range: 264-916 ng/dL Optimal for vitality and longevity: 500-900 ng/dL Free testosterone matters more — 2-3% of total testosterone is unbound and biologically active. Request free testosterone and SHBG alongside total T.

Vitamin D (25-OH)

Reference range: 30-100 ng/mL Optimal: 40-60 ng/mL based on association studies with lowest all-cause mortality Very common deficiency — 42% of US adults have levels below 20 ng/mL.

FAQ

What should I ask my doctor to add to standard bloodwork?

Standard annual labs miss many metabolically important markers. Additions worth requesting: ApoB (or LDL-P via NMR LipoProfile), fasting insulin, free testosterone + SHBG (men), 25-OH Vitamin D, hsCRP (high-sensitivity C-reactive protein for inflammation), homocysteine (cardiovascular risk factor), uric acid. Most are covered by insurance with appropriate ICD-10 codes.

What does it mean when something is slightly out of range?

A single slightly abnormal result usually doesn't require immediate action. Context matters: direction (moving toward or away from normal), magnitude (10% vs 300% above range), symptoms, and trend over time. Always retest before making treatment decisions.

Should I fast before a blood draw?

For accurate fasting glucose, insulin, and triglycerides: yes, 10-12 hours fasting. Most other tests (CBC, CMP electrolytes, thyroid, cholesterol/LDL) are minimally affected by fasting. If you're doing a comprehensive panel, fasting for the whole draw is simplest.

Related guides: Blood Work Biomarkers for Longevity | Best At-Home Blood Tests | Intermittent Fasting and Metabolic Health

Updated March 2026