Clinical Chemistry and Laboratory Medicine
Overview
This domain tests whether you can interpret a chemistry panel the way a laboratorian does: knowing reference intervals, analytical and biological variation, interference, and the performance characteristics of the assays themselves.
Core rules
- Reference intervals are usually central 95% intervals; a result outside them is not automatically disease, and a result inside them does not exclude disease.
- Troponin: high-sensitivity assays use the sex-specific 99th percentile upper reference limit; interpretation requires the delta change between serial samples.
- Anion gap = Na − (Cl + HCO3), normally 8–12 mmol/L; a normal-gap acidosis suggests bicarbonate loss or renal tubular acidosis.
- Osmolal gap = measured − calculated osmolality; elevated in methanol, ethylene glycol and isopropanol ingestion.
- Interference: hemolysis raises potassium, LDH and AST; lipemia causes pseudohyponatremia in indirect ion-selective methods; icterus interferes with creatinine assays. Automated HIL indices flag these.
- HbA1c reflects about three months of glycemia but is unreliable in hemoglobinopathies, recent transfusion and altered red cell survival.
Worked example
A patient with severe hypertriglyceridemia has Na 121 mmol/L but no neurologic symptoms. Reasoning: in indirect ion-selective electrode methods, the aqueous fraction of plasma is reduced by excess lipid, producing pseudohyponatremia. Direct ion-selective measurement or a blood gas sodium resolves it. Treating the number with hypertonic saline would be harmful.
Traps
Ignoring sample quality flags. Using the Friedewald calculation when triglycerides exceed 400 mg/dL. Treating a single troponin as diagnostic of infarction without a serial delta.