Clinical Chemistry: Analytic Principles, Calculations, and Interference
The chemistry portion tests analytic principles (Beer-Lambert, calibration, QC), calculations performed at the bench, and the pathophysiology behind abnormal results.
Analytic basics
Absorbance is proportional to concentration (A = εbc); results outside the linear range must be diluted, re-run, and corrected by the dilution factor. Reference intervals are usually the central 95% of a healthy population, derived from at least 120 reference individuals. Westgard multirules combine random-error (1-3s) and systematic-error (2-2s, 4-1s, 10x) detection; a 1-2s warning rule alone is not grounds for rejection.
Calculations worth memorizing
Anion gap = Na − (Cl + HCO3), typically 8–12 mmol/L; correct for hypoalbuminemia by adding 2.5 × (4.0 − albumin). Calculated osmolality = 2Na + glucose/18 + BUN/2.8; a measured-to-calculated gap above 10 suggests an unmeasured osmole. The 2021 race-free CKD-EPI equation uses creatinine, age, and sex only. HbA1c is reported as NGSP percent or IFCC mmol/mol, and the two are not interchangeable.
Interference
Hemolysis falsely raises potassium, LDH, and AST. Lipemia scatters light in photometric assays; icterus interferes at short wavelengths. High-dose biotin competes in streptavidin-biotin immunoassays and can bias results in either direction depending on format.
Worked example
Sodium 138, chloride 108, bicarbonate 14, albumin 2.0. Anion gap = 16; albumin-corrected gap = 16 + 2.5 × (4.0 − 2.0) = 21. This is a high anion gap metabolic acidosis, and the correction matters because the uncorrected value in a hypoalbuminemic patient can understate a real acidosis.
Traps that cost marks
EDTA contamination from a lavender tube drawn before chemistry tubes raises potassium and lowers calcium. Reporting ionized calcium from a heparinized tube without accounting for heparin binding. Confusing analytic total error with biological variation when judging whether a delta is real.