Clinical Chemistry
Core concepts
Clinical chemistry questions reward a disciplined approach: know the analyte, its organ of origin, its half-life, and the pre-analytical variables that distort it. Master the aminotransferases and their ratio, alkaline phosphatase with GGT to localise hepatobiliary versus bone origin, bilirubin fractionation, and the patterns of renal function testing (creatinine, urea, cystatin C, eGFR equations). Understand acid-base interpretation using the Henderson-Hasselbalch relationship, the anion gap, and compensation rules (Winter's formula for metabolic acidosis). Electrolytes demand fluency in the sodium-water axis (osmolality, tonicity, corrected sodium in hyperglycaemia) and potassium shifts.
Traps in questions
Haemolysis falsely raises potassium, LDH and AST while lowering haptoglobin. Delayed sample separation raises glucose and potassium and lowers bicarbonate. Do not accept a normal anion gap as excluding metabolic acidosis - hyperchloraemic acidosis is normo-gap by definition. Watch for the difference between osmolality (measured) and osmolarity (calculated) and the osmolal gap in toxic alcohol ingestion. Cardiac troponin interpretation requires the 99th percentile upper reference limit and a serial change (delta), not a single value.
Worked example
A patient with vomiting has pH 7.50, PaCO2 48 mmHg, HCO3 36 mmol/L. The pH is alkalotic; the raised bicarbonate indicates a primary metabolic alkalosis; the raised CO2 is appropriate respiratory compensation (expected PaCO2 rises roughly 0.7 mmHg per 1 mmol/L rise in HCO3). If PaCO2 were lower than expected, a superimposed respiratory alkalosis would be present. Always state the primary disorder, the compensation, and any second disorder.
Rule of thumb
For any abnormal result, ask: is this pre-analytical, physiological, or pathological? Correct the sample before correcting the patient.