Clinical Chemistry
Preanalytical control is where questions hide
Most wrong chemistry answers begin before the sample is analyzed. Hemolysis falsely raises potassium, LDH, AST, and phosphate and interferes with bilirubin assays. Prolonged tourniquet time hemoconcentrates protein, calcium, and potassium. EDTA contamination from a lavender tube drawn before a chemistry tube produces striking hyperkalemia with hypocalcemia and low ALP. Ammonia and blood gas specimens require ice and rapid transport.
Core calculations you must own
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); reference roughly 8–12 mmol/L.
- Calculated osmolality = 2Na + glucose/18 + BUN/2.8; an osmolar gap >10 suggests an unmeasured osmole.
- Corrected calcium = measured Ca + 0.8 × (4.0 − albumin g/dL).
- BUN (mg/dL) × 0.357 = urea (mmol/L).
Organ panels
Know the patterns: hepatocellular injury (AST/ALT, ALT > AST in viral, AST/ALT > 2 in alcohol), cholestasis (ALP and GGT rise together; ALP alone may be bone), renal function (creatinine is muscle-mass dependent, cystatin C is not), and cardiac markers (high-sensitivity troponin with serial delta changes).
Traps
Do not confuse analytical sensitivity with clinical sensitivity; do not interpret a value inside the reference interval as normal for every patient. Watch for lipemia and icterus as interference, and remember delta checks and critical value callbacks are required, not optional.
Worked example: Na 140, Cl 105, HCO₃ 18 → anion gap 17, a high-anion-gap metabolic acidosis. If the delta ratio (ΔAG / ΔHCO₃) is ≈1, pure high-AG acidosis; <0.4 suggests a coexisting normal-AG acidosis.