Clinical Chemistry
Core concepts
Chemistry questions test analytical reasoning as much as memorized ranges. Know the Beer-Lambert relationship behind spectrophotometry, the difference between end-point and kinetic (rate) methods, and how ion-selective electrodes measure sodium, potassium, and chloride. Be fluent with renal markers (creatinine, cystatin C, BUN), the anion gap, osmolality and the osmolal gap, albumin-corrected calcium, calculated LDL, HbA1c with estimated average glucose, and blood gas interpretation using the Henderson-Hasselbalch relationship.
Traps
- Hemolysis falsely raises potassium, LDH, and AST and interferes with many colorimetric assays; lipemia and icterus scatter or absorb light. Modern analyzers flag these as HIL indices.
- The anion gap is Na minus (Cl + HCO3), normally about 8-12 mmol/L. Hypoalbuminemia lowers the gap, so add 2.5 x (4.0 - albumin) to the measured value.
- Glucose falls in uncentrifuged tubes because cells consume it; fluoride oxalate slows but does not stop glycolysis.
- Reference intervals are method- and population-dependent, so a value just outside the interval is not automatically abnormal.
Worked example
Na 138, Cl 102, HCO3 12, albumin 2.0 g/dL. Measured gap = 24 mmol/L. Corrected gap = 24 + 2.5 x (4.0 - 2.0) = 29 mmol/L, a high-anion-gap acidosis that the uncorrected number would have underestimated.