Clinical Biochemistry
Clinical biochemistry in the NHPC BMLT paper tests whether you can select the right specimen, understand the analytical principle behind a method, and interpret a panel against method-dependent reference intervals. Expect acid–base problems, renal and liver profiles, enzymology, glucose and lipid disorders, and pre-analytical errors that change results.
Core rules
- Use fluoride–oxalate for glucose; fluoride blocks enolase and slows glycolysis, whereas plain serum glucose falls several percent per hour at room temperature.
- Hemolysis releases potassium, LDH and AST and dilutes other analytes; EDTA contamination raises potassium and lowers calcium.
- Beer–Lambert law: absorbance = molar absorptivity × concentration × path length, so absorbance is linear with concentration only within the assay's validated range.
- Anion gap = Na − (Cl + HCO3), normally 8–12 mmol/L.
Recognising patterns
Hepatocellular injury gives ALT greater than AST, often above 500 U/L in acute viral or drug hepatitis, while an AST:ALT ratio above 2 suggests alcohol. Cholestasis raises ALP and GGT together; an isolated ALP may be bone in origin, so check GGT. Iron deficiency shows low ferritin, high TIBC and low transferrin saturation.
Worked example
A young diabetic presents with vomiting: Na 140, Cl 104, HCO3 14, glucose 28 mmol/L, pH 7.22, pCO2 26. Anion gap = 140 − 118 = 22, so this is a high-gap metabolic acidosis with appropriate respiratory compensation. The picture fits diabetic ketoacidosis. Calculate the delta ratio, (22 − 12)/(24 − 14) = 1.0, showing no second acidosis; a ratio below 1 would suggest coexisting normal-gap loss of bicarbonate, such as diarrhoea.
Traps that cost marks
Reporting results from a visibly hemolysed sample, quoting a reference interval borrowed from another analyser, and forgetting that delayed centrifugation falsely raises potassium and phosphate.