Clinical Chemistry and Laboratory Medicine: Valid Results
Clinical Chemistry and Laboratory Medicine tests preanalytical, analytical and postanalytical reasoning: reference intervals, sensitivity and specificity, quality control, renal and liver panels, cardiac markers, electrolytes and acid-base disorders.
Core rules
- Preanalytical error is common: hemolysis raises potassium and LDH; lipemia interferes with some assays; prolonged tourniquet raises potassium and protein.
- Reference intervals describe a central portion of a healthy population; they are not treatment thresholds.
- Sensitivity detects disease when present; specificity excludes disease when absent. Predictive value depends on prevalence.
- Quality control uses control materials and statistical rules; a shift suggests calibration or reagent change, not random error.
- Anion gap = sodium minus chloride minus bicarbonate; albumin correction matters.
- Cardiac troponin is preferred for myocardial injury; a rise and fall with clinical evidence supports infarction.
Worked example
A patient has sodium 140 mmol/L, chloride 100 mmol/L, bicarbonate 14 mmol/L and glucose 6 mmol/L. The anion gap is 26 mmol/L, indicating a high-anion-gap metabolic acidosis. Reasoning: bicarbonate is low, sodium is normal and chloride is normal, so unmeasured anions are increased. Check lactate, ketones, renal function and toxins. If albumin is low, the corrected gap is higher than calculated.
Traps
Do not interpret potassium without excluding hemolysis or pseudohyperkalemia. Correct calcium for albumin. Do not use a single troponin value to diagnose infarction without timing and clinical context. A normal QC result does not prove every patient result is correct.