Clinical Chemistry: Analyte Behavior, Interferences, and Calculations
Core concepts
Understand the major analyte groups and their physiology: electrolytes and osmolality, renal markers (BUN, creatinine, eGFR), liver enzymes (ALT, AST, ALP, GGT, bilirubin), cardiac markers (troponin, CK-MB, BNP), glucose regulation (fasting glucose, HbA1c, insulin), and thyroid hormones (TSH, free T4). Know which enzymes are hepatocellular (ALT/AST) versus cholestatic (ALP/GGT). Understand reference intervals and how they differ by age, sex, and pregnancy.
Interferences and quality
Know the classic interferences: hemolysis (falsely high K, LDH, AST; falsely low glucose), lipemia (light-scattering effects on photometry), icterus, and drugs. Know the difference between accuracy and precision, and how calibration and QC (Levey-Jennings, Westgard rules) detect systematic vs random error. Understand the anion gap (Na − [Cl + HCO3]) and its use in metabolic acidosis, and the concept of osmolal gap.
Traps
Questions often ask which result is falsely elevated in hemolysis (potassium) or which enzyme is most specific for liver (ALT). Watch for unit conversions and for the difference between total and direct bilirubin. Also beware confusing analytical sensitivity (limit of detection) with clinical sensitivity.
Worked example
Na 140, Cl 100, HCO3 20 → anion gap = 140 − 120 = 20 mmol/L (elevated; normal ~8–12). This suggests an anion-gap metabolic acidosis (e.g., ketoacidosis, lactic acidosis, toxin). Rule of thumb: always compute the gap before interpreting a bicarbonate.