Clinical Chemistry
What to master
Chemistry questions test whether you can trace a number back to preanalytical and analytical causes. Master spectrophotometry and Beer's law, then distinguish reference intervals, decision limits, and critical values. Renal: creatinine, cystatin C, eGFR equations, and the BUN/creatinine ratio (prerenal azotemia above roughly 20:1). Electrolytes: anion gap = Na − (Cl + HCO3); osmolal gap = measured − calculated osmolality (2Na + glucose/18 + BUN/2.8). Liver: hepatocellular pattern (ALT and AST; AST greater than ALT with alcohol, ALT greater than AST with viral hepatitis) versus cholestatic pattern (ALP and GGT), plus bilirubin metabolism. Cardiac troponin with serial delta values. Lipids with Friedewald (LDL = TC − HDL − TG/5, invalid when TG exceeds 400 mg/dL). HbA1c method-dependent interference from hemoglobin variants. Thyroid screening with TSH. Drug monitoring timing. Interference: hemolysis (raises potassium, LDH, AST), icterus, lipemia, biotin in streptavidin immunoassays, and hyperproteinemia causing pseudohyponatremia on indirect ion-selective electrodes.
Traps
EDTA contamination inflates potassium and lowers calcium; fluoride tubes preserve glucose but not all analytes; delayed centrifugation lets potassium leak from cells while glucose falls artifactually.
Worked example
Corrected calcium = measured Ca + 0.8 × (4.0 − albumin). A patient with Ca 8.0 mg/dL and albumin 2.0 g/dL: 0.8 × 2.0 = 1.6, so corrected calcium is 9.6 mg/dL, which is normal despite the low measured value.