Clinical Chemistry
Clinical Chemistry
This domain tests spectrophotometry, enzyme-based assays, electrolyte and acid-base calculations, and recognition of analytic interference - the reasoning behind a chemistry result rather than the instrument's menu.
Core rules
- Beer-Lambert law: absorbance is proportional to concentration and path length. Assays must stay in the linear range; above it, dilute and re-run with a dilution factor.
- Enzymatic methods measure rate of NADH formation or consumption at 340 nm; lag and linear phases must be distinguished before rate readings are taken.
- The anion gap is Na − (Cl + HCO3), normally about 8–12 mmol/L without potassium. Correct for hypoalbuminemia by adding 2.5 × (4.0 − albumin g/dL).
- The delta ratio, (measured AG − 12)/(24 − measured HCO3), separates pure high-anion-gap acidosis from mixed disorders.
- Hemolysis falsely raises potassium, LDH, AST, and phosphate, and lowers haptoglobin; lipemia and icterus interfere with several photometric methods.
Worked example
Na 136, Cl 102, HCO3 10, albumin 2.0 g/dL. Anion gap = 136 − 112 = 24. Albumin correction adds 2.5 × 2.0 = 5, giving 29 - a high-anion-gap metabolic acidosis. Delta ratio = (29 − 12)/(24 − 10) = 17/14 ≈ 1.2, consistent with a pure high-AG acidosis (roughly 1–2); values below 1 suggest a concurrent normal-AG acidosis.
Traps that cost marks
- Reporting a critically high potassium drawn from a hemolyzed or EDTA-contaminated tube.
- Using the uncorrected anion gap in a hypoalbuminemic patient.
- Failing to dilute a result flagged above the analytic range.
- Confusing plasma and serum reference intervals for potassium and glucose.