Memory tricks for normal ranges and what they mean

WBC (4,500–11,000/µL): ↑ = infection, inflammation, leukemia. ↓ = bone marrow suppression, viral illness, chemotherapy. Differential: neutrophils (55–70%), lymphocytes (20–40%), monocytes (2–8%), eosinophils (1–4%), basophils (0.5–1%). RBC (4.5–5.5 million/µL M, 4.0–5.0 F): ↓ = anemia. ↑ = polycythemia vera, dehydration. Hemoglobin (12–17 g/dL): ↓ = anemia (pale, fatigue, dyspnea). Hematocrit (37–52%): ≈ 3× hemoglobin. Platelets (150,000–400,000/µL): ↓ = thrombocytopenia → bleeding risk. ↑ = thrombocytosis → clotting risk. MCV (80–100 fL): ↓ = microcytic (iron deficiency), ↑ = macrocytic (B12/folate deficiency).

Sodium (135–145 mEq/L): ↓ hyponatremia (SIADH, heart failure, over-hydration) → confusion, seizures. ↑ hypernatremia (dehydration, DI) → thirst, dry mucous membranes. Potassium (3.5–5.0 mEq/L): ↓ hypokalemia → muscle weakness, arrhythmias (U wave on ECG). ↑ hyperkalemia → peaked T waves → cardiac arrest. Creatinine (0.6–1.2 mg/dL): kidney waste product — best simple kidney marker. ↑ = kidney disease or decreased GFR. BUN (7–20 mg/dL): urea nitrogen. BUN:Cr ratio >20 = prerenal. Glucose (70–100 fasting): ↑ = diabetes. ↓ = hypoglycemia → tremor, diaphoresis, confusion. Bicarbonate (22–29 mEq/L): metabolic acid-base marker.

Step 1 — pH: <7.35 = acidosis. >7.45 = alkalosis. Step 2 — Respiratory (PaCO₂): ↑ CO₂ = more acid → respiratory acidosis. ↓ CO₂ = more alkaline → respiratory alkalosis. CO₂ moves OPPOSITE to pH (ROME-R). Step 3 — Metabolic (HCO₃⁻): ↑ HCO₃⁻ = more alkaline → metabolic alkalosis. ↓ HCO₃⁻ = more acid → metabolic acidosis. HCO₃⁻ moves SAME direction as pH (ROME-M). Step 4 — Compensation: Lungs compensate metabolic disorders (minutes). Kidneys compensate respiratory disorders (days). PaO₂ 80–100 mmHg: <60 = hypoxemia → supplemental O₂ needed. SpO₂ = peripheral O₂ saturation (pulse oximeter) — normal >95%.

Troponin I and T: most sensitive and specific cardiac marker. Rises 3–6 hours post-MI, peaks 12–24 hours, remains elevated 7–14 days. High-sensitivity troponin (hs-cTn) detectable within 1 hour. Any elevation = myocardial injury. CK-MB (creatine kinase MB isoform): rises 4–6 hours, peaks 12–24 hours, returns to normal 2–3 days. Useful for detecting reinfarction (troponin stays elevated). BNP / NT-proBNP: released by ventricles when stretched. ↑ = heart failure. Normal BNP <100 pg/mL makes heart failure unlikely. Used to diagnose and monitor CHF. Myoglobin: earliest marker (1–2 hours) but not specific to heart. LDH: late marker, peaks 3–5 days, stays elevated 10 days — historical.

ALT (alanine aminotransferase, 7–56 U/L): most specific for liver — rises in hepatocyte damage (hepatitis, cirrhosis, fatty liver, drug toxicity). AST (aspartate aminotransferase, 10–40 U/L): less specific (also heart, muscle). AST:ALT ratio >2:1 = alcoholic hepatitis. ALP (alkaline phosphatase, 44–147 U/L): ↑ = cholestasis (bile duct obstruction), bone disease, pregnancy. GGT confirms biliary source. Bilirubin (0.1–1.2 mg/dL total): unconjugated (indirect) ↑ = hemolysis or liver uptake failure. Conjugated (direct) ↑ = hepatic or post-hepatic obstruction. Jaundice visible >2.5 mg/dL. Albumin (3.5–5.0 g/dL): synthetic function — ↓ = liver failure, malnutrition. PT/INR: clotting factor synthesis — ↑ INR = liver failure or warfarin.

Creatinine (0.6–1.2 mg/dL): muscle waste product, freely filtered — best simple GFR estimate. Doubles when GFR halves. Rises slowly — takes 24–48 hours to reflect acute injury. BUN (7–20 mg/dL): blood urea nitrogen — affected by protein intake, hydration, GI bleeding. eGFR (estimated GFR): calculated from creatinine + age + sex + race. Normal >60 mL/min. CKD defined as eGFR <60 for >3 months. BUN:Creatinine ratio: >20 = prerenal (dehydration, GI bleeding, heart failure — kidney OK but underperfused). <10 = intrinsic renal (ATN, glomerulonephritis). 10–20 = normal. Urine sodium: <20 mEq/L = prerenal (kidney conserving Na+). >40 mEq/L = intrinsic renal (tubules can't reabsorb). FENa = fractional excretion of sodium — <1% prerenal, >2% intrinsic.

TSH (thyroid-stimulating hormone, 0.4–4.0 mU/L): most sensitive thyroid test — reflects pituitary response to T3/T4. High TSH = pituitary working hard to stimulate failing thyroid = hypothyroidism. Low TSH = pituitary suppressed by excess thyroid hormone = hyperthyroidism. Free T4 (0.8–1.8 ng/dL): unbound active form. Confirms and grades severity. Primary hypothyroidism: ↑ TSH + ↓ T4. Treat with levothyroxine. Primary hyperthyroidism: ↓ TSH + ↑ T4. Graves' disease most common. Secondary hypothyroidism (pituitary failure): ↓ TSH + ↓ T4. Subclinical hypothyroidism: ↑ TSH + normal T4 — treat if TSH >10 or symptomatic.

PT (prothrombin time, 11–13 sec) / INR (0.8–1.1): tests extrinsic pathway (factor VII) and common pathway. Prolonged by: warfarin (blocks vitamin K-dependent factors VII, IX, X, II), liver failure (can't make factors), vitamin K deficiency. INR 2–3 = therapeutic warfarin range for most indications. PTT (partial thromboplastin time, 25–35 sec): tests intrinsic pathway and common pathway. Prolonged by: heparin, hemophilia A (factor VIII) or B (factor IX), lupus anticoagulant. Therapeutic heparin PTT = 60–100 sec. Both prolonged: liver failure, DIC, massive transfusion. D-dimer (normal <0.5 µg/mL): fibrin degradation product → ↑ in DVT/PE, DIC, any clot. High sensitivity but low specificity. Normal D-dimer rules out PE (high sensitivity).

Fasting plasma glucose: Normal <100 mg/dL. Prediabetes 100–125 mg/dL. Diabetes ≥126 mg/dL (confirmed on 2 occasions). Random glucose ≥200 mg/dL with symptoms = diabetes. 2-hour OGTT (oral glucose tolerance test): normal <140 mg/dL. Prediabetes 140–199 mg/dL. Diabetes ≥200 mg/dL. HbA1c (glycated hemoglobin): reflects average glucose for ~3 months (RBC lifespan). Normal <5.7%. Prediabetes 5.7–6.4%. Diabetes ≥6.5%. Target for diabetics: <7% (ADA). Each 1% HbA1c ≈ average glucose of 28 mg/dL. Insulin and C-peptide: ↑ both = Type 2 or insulinoma. ↓ both = Type 1 (no insulin production). ↑ insulin + ↓ C-peptide = exogenous insulin injection (factitious hypoglycemia).