Clinical Chemistry
What this domain tests
Clinical Chemistry questions ask you to choose and defend a measurement method, keep it in control, and interpret the result against the patient. Expect photometry, enzyme kinetics, immunoassay design, interference, and organ-system interpretation (renal, cardiac, hepatic, endocrine, diabetes).
Analytical foundations
- Beer–Lambert (A = εbc) underpins photometry; linearity fails at high absorbance from stray light, so every dilution scheme must be validated.
- Enzyme activity is a rate (U/L = micromoles of substrate converted per minute per liter). NAD(H)-linked reactions are read at 340 nm. Reference intervals are temperature-dependent: IFCC 30 °C and 37 °C methods are not interchangeable.
- Immunoassay formats: sandwich assays for large analytes with excess antibody, competitive assays for small haptens. Very high antigen can saturate both formats - the hook effect in sandwich assays gives a falsely low result.
- Interference indices (hemolysis, icterus, lipemia) are screening flags, not diagnoses.
Interpretation anchors
- Renal: IDMS-traceable creatinine with the CKD-EPI 2021 race-free eGFR equation; cystatin C as a confirmatory marker.
- Cardiac: high-sensitivity troponin is judged against the sex-specific 99th percentile upper reference limit plus a serial delta; a single value near the URL is not diagnostic.
- Thyroid: TSH is first-line; free T4 by equilibrium dialysis when binding proteins are abnormal (pregnancy, cirrhosis, dysalbuminemia).
- Diabetes: HbA1c must be NGSP-certified; report both NGSP % and IFCC mmol/mol. Hemoglobin variants and anemia distort results.
Worked example
Sodium 128, chloride 100, bicarbonate 22, albumin 2.4 g/dL. Measured anion gap = 128 − 100 − 22 = 6 mmol/L, which looks low. Correcting for albumin: add 2.5 × (4.0 − 2.4) = 4.0, giving a corrected gap of 10. Reasoning: each 1 g/dL fall in albumin lowers the measured gap by roughly 2.5, so without correction a genuine high-gap acidosis can be hidden. A gap of 6 with a normal albumin would have suggested a different differential.
Traps that cost marks
- Reporting a hemolyzed potassium as physiologic; hemolysis raises K, LDH and AST and dilutes sodium.
- Quoting an enzyme result without the method temperature.
- Confusing the delta gap or delta ratio with the anion gap itself.
- Treating a moderate lipemic index as harmless when the analyte is measured by light scatter.
- Using total calcium without albumin adjustment in a hypoalbuminemic patient.