Clinical Chemistry
Analytical principles and interferences
Clinical chemistry questions reward understanding of spectrophotometry, the Beer-Lambert law, reagent stability, calibration, and interference. Hemolysis releases potassium, LDH, and AST; it also interferes with many colorimetric assays. Icterus and lipemia cause spectral and chemical interference. Glucose falls in tubes without glycolysis inhibition if separation is delayed; fluoride tubes slow but do not stop glycolysis indefinitely. Potassium is the most common hemolysis-sensitive analyte. Creatinine and BUN assess renal function, but BUN is affected by hydration, catabolism, and GI bleeding; the BUN/creatinine ratio helps interpret prerenal states. Electrolytes require attention to pseudohyponatremia in severe hyperlipidemia or hyperproteinemia and to sample contamination. Liver enzymes (ALT, AST, ALP, GGT, bilirubin) and cardiac troponin have specific release kinetics. Reference intervals are method- and population-dependent.
Traps and worked rule
A frequent trap is reporting hyperkalemia in a hemolyzed sample; another is a normal anion gap with a low bicarbonate, suggesting bicarbonate loss rather than an unmeasured anion. Worked example: anion gap = Na − (Cl + HCO3). If Na 140, Cl 105, HCO3 15, gap = 20 mmol/L, suggesting an anion-gap metabolic acidosis; check lactate, ketones, renal failure, and toxins. Rule of thumb: when a chemistry result conflicts with the clinical picture, check specimen integrity, turnaround delay, and dilution before repeating.