Clinical Chemistry
Measurement and QC
Photometric methods obey Beer-Lambert (A = εbc), so linearity and reagent blanks matter. Levey-Jennings charts and Westgard rules (1-3s, 2-2s, R-4s, 4-1s, 10x) separate random from systematic error. Reference intervals describe a healthy population; decision limits (such as glucose) drive clinical action.
Electrolytes and acid-base
Sodium by indirect ISE can be falsely low in hyperlipidemia or hyperproteinemia (pseudohyponatremia); direct ISE avoids this. Anion gap = Na − (Cl + HCO3), reference 8–12 mmol/L; add ~2.5 mmol/L to the gap for every 1.0 g/dL albumin below 4.0. Osmolal gap = measured − (2Na + glucose/18 + BUN/2.8).
Organ panels
Liver: AST/ALT ratio >2 suggests alcohol-related injury; alkaline phosphatase with elevated GGT points to cholestasis. Cardiac troponin I/T are preferred over CK-MB, interpreted with serial change. TSH is the first-line thyroid screen. Creatinine-based eGFR (CKD-EPI) is standard, with cystatin C when muscle mass is atypical. HbA1c reflects roughly 3 months of glycemia.
Traps
- Ignoring albumin when interpreting the anion gap.
- Corrected calcium for low albumin (add 0.8 mg/dL per 1.0 g/dL below 4.0).
- Biotin and heterophile antibodies interfering with immunoassays.
- Failing to use the delta gap/delta ratio in mixed acid-base disorders.
Worked example: Na 138, Cl 105, HCO3 24, albumin 2.0 → AG = 9; corrected = 9 + 2.5 × 2 = 14, unmasking a high anion gap acidosis.