Clinical Chemistry
Core concepts
Clinical chemistry questions reward a disciplined approach: know the analyte, its pre-analytic pitfalls, and the physiologic logic behind its change. Master acid–base interpretation (Henderson–Hasselbalch, anion gap, delta gap, Winter's formula for expected pCO2), electrolyte disorders (corrected sodium for hyperglycemia, osmolar gap), and the urea/creatinine relationship in prerenal azotemia. Understand cardiac markers (troponin kinetics and specificity), liver enzyme patterns (hepatocellular vs cholestatic, R-factor), and tumor markers as monitoring rather than screening tools. Know endocrine testing principles: dynamic suppression/stimulation tests, free hormone concepts, and assay interference (biotin, heterophile antibodies, macroenzymes).
Traps
Questions love pre-analytic error: hemolysis falsely raising potassium and LDH, prolonged tourniquet raising potassium and protein, EDTA contamination raising potassium and lowering calcium, and delayed separation raising glucose via glycolysis. Watch for spurious hyperkalemia with normal renal function and no ECG changes. Another trap is confusing the anion gap causes (MUDPILES vs normal-gap acidosis from GI or renal bicarbonate loss). Correct sodium for glucose before interpreting dysnatremia. Remember that a normal anion gap does not exclude a toxic ingestion.
Worked example / rule of thumb
For a mixed acid–base picture, calculate the delta ratio: (measured AG − 12)/(24 − measured HCO3). A value <1 suggests concurrent normal-gap acidosis; >2 suggests concurrent metabolic alkalosis. Rule of thumb: always compute the expected compensation before calling a disorder "mixed."