Memory tricks for normal ranges and what they mean
Lab values connect anatomy to clinical medicine. Knowing normal ranges — and what happens when they fall outside them — helps you understand which organ system is involved. These memory tricks help the numbers stick.
3-month average glucose. <5.7% normal. 5.7–6.4% pre-diabetes. ≥6.5% diabetes.
Liver Function Tests — LFTs
ALT AST ALP — All Liver Tests · ALT most specific for liver
ALT · AST · ALP · Bilirubin · Albumin · PT/INR
Liver function test panel — what each value tells you about the liver
ALT (alanine aminotransferase): most specific for liver damage. Normal 7–56 U/L. AST (aspartate aminotransferase): liver and heart/muscle. Normal 10–40 U/L. AST/ALT ratio >2:1 suggests alcoholic hepatitis. ALP (alkaline phosphatase): liver and bone. High in cholestasis and Paget's disease. Bilirubin: total 0.1–1.2 mg/dL. High = jaundice. Albumin: 3.5–5.0 g/dL. Low = chronic liver disease or malnutrition. PT/INR: measures clotting factor synthesis — elevated in liver failure.
ALT
Most liver-specific. High = hepatitis, fatty liver, drug toxicity.
High HCO3 — vomiting, nasogastric suction, diuretic use.
Cardiac Markers
Troponin = Gold standard · CK-MB = Early · BNP = Heart failure
Troponin I/T · CK-MB · BNP/NT-proBNP · Myoglobin
Cardiac biomarkers — what each one indicates and when it peaks
Troponin I and T: most sensitive and specific for myocardial infarction. Rises 3–6 hours after MI, peaks 24 hours, remains elevated 7–14 days. Gold standard for MI diagnosis. CK-MB: rises 3–6 hours, peaks 24 hours, returns to normal in 3 days — useful for detecting reinfarction. BNP/NT-proBNP: released by ventricular myocytes under pressure — elevated in heart failure. BNP >100 pg/mL suggests heart failure. Myoglobin: rises earliest (1–3 hours) but not specific for heart — also elevated in muscle injury.
Troponin
Gold standard MI marker. Rises 3–6 hr, peaks 24 hr, stays elevated 7–14 days.
CK-MB
Returns to normal in 3 days — useful for detecting re-infarction after initial MI.
BNP
>100 pg/mL = heart failure likely. >400 pg/mL = strong evidence. Used to rule out HF.
Myoglobin
First to rise (1–3 hr) but not heart-specific. Elevated in any muscle injury.
Thyroid Labs
High TSH + Low T4 = Hypothyroid · Low TSH + High T4 = Hyperthyroid
TSH 0.4–4.0 mIU/L · Free T4 0.8–1.8 ng/dL
Thyroid lab interpretation — the inverse relationship that explains everything
TSH is the most sensitive thyroid test — it changes before T4 does. Normal TSH: 0.4–4.0 mIU/L. Normal free T4: 0.8–1.8 ng/dL. High TSH + Low T4 = Primary hypothyroidism (thyroid gland failing). Low TSH + High T4 = Hyperthyroidism (too much thyroid hormone feeding back to suppress TSH). Low TSH + Low T4 = Central hypothyroidism (pituitary/hypothalamus problem). TSH is always the first test ordered — a normal TSH essentially rules out thyroid disease.
High TSH + Low T4
Primary hypothyroidism — thyroid gland not responding. Treat with levothyroxine.
Low TSH + High T4
Hyperthyroidism — excess hormone suppresses TSH. Graves' disease most common.
High TSH + Normal T4
Subclinical hypothyroidism — early, may progress to overt hypothyroidism.
Low TSH + Low T4
Central hypothyroidism — pituitary or hypothalamus not signaling thyroid.
PT tests extrinsic pathway · PTT tests intrinsic pathway
Coagulation tests — which clotting factors each one measures
PT (prothrombin time): measures extrinsic pathway. Factors I, II, V, VII, X. Normal 11–13 seconds. INR normalizes PT between labs — normal 0.8–1.2. Elevated in liver disease and warfarin use. PTT (partial thromboplastin time): measures intrinsic pathway. Factors VIII, IX, XI, XII. Normal 25–35 seconds. Elevated in hemophilia (factor VIII or IX deficiency) and heparin use. Bleeding time: platelet function. Thrombin time: fibrinogen function.
PT/INR elevated
Liver disease (can't make clotting factors) or warfarin. Extrinsic pathway.
PTT elevated
Hemophilia A (VIII) or B (IX), or heparin therapy. Intrinsic pathway.
Both elevated
DIC (disseminated intravascular coagulation) or severe liver failure.
Three types of anemia by MCV — the fastest way to narrow the diagnosis
MCV (mean corpuscular volume) is the key to classifying anemia. Microcytic (MCV <80): small red cells. Causes: iron deficiency (most common), thalassemia, lead poisoning, chronic disease. Remember: ITS MCV — Iron deficiency, Thalassemia, Sideroblastic, Microcytic. Normocytic (MCV 80–100): normal size. Causes: acute blood loss, hemolysis, anemia of chronic disease, early iron deficiency. Macrocytic (MCV >100): large red cells. Causes: B12 deficiency, folate deficiency, liver disease, hypothyroidism, alcohol.
Microcytic <80
Iron deficiency (most common globally). Low ferritin. High TIBC.
Low ferritin, low iron, high TIBC. Most common anemia worldwide.
B12 deficiency
Low B12, elevated MMA and homocysteine. Neurological damage if untreated.
📋 Study note: Lab value ranges shown here are standard reference ranges used in medical education. Normal ranges can vary slightly between laboratories and institutions. These values are for learning purposes — always refer to your course materials and institutional references for clinical application.
📝 Exam Prep
5 Common Exam Questions
Frequently tested concepts — know these cold before your exam.
❓ What are the components of a CBC and what does each measure?
✅ WBC (immune cells, normal 4,500–11,000), RBC (oxygen carriers), Hemoglobin (O2-carrying protein in RBCs), Hematocrit (% of blood that is RBCs), MCV (RBC size), Platelets (clotting, normal 150,000–400,000). Differential breaks down WBC types.
❓ What is the BMP and what conditions does it help diagnose?
✅ BMP = Na, K, Cl, CO2, BUN, Creatinine, Glucose, Calcium. Used to assess kidney function (BUN/Cr), electrolyte balance, acid-base status, and glucose. BUN:Cr ratio >20 suggests pre-renal azotemia.
❓ How do you interpret an ABG — what are the normal values?
❓ What troponin level indicates myocardial infarction?
✅ Any troponin above the 99th percentile of the reference population. Serial troponins (at 0, 3, 6 hours) are used — a rising and/or falling pattern confirms acute MI. High-sensitivity troponin can detect MI within 1–2 hours.
❓ What does an elevated INR indicate and when is it dangerous?
✅ Elevated INR means blood takes longer to clot. Normal INR = 1.0. Therapeutic range for warfarin = 2.0–3.0 (2.5–3.5 for mechanical valves). INR >4 = significantly elevated bleeding risk. INR >5 with bleeding = treat with Vitamin K or FFP.