Standards and Units of Measurement
Candidates must convert confidently between mass and substance (molar) units and recognize SI versus conventional reporting. The core relationship is concentration in mass units divided by molecular weight equals molar concentration. Know the molecular weights commonly tested: glucose 180 g/mol, urea 60 (but urea nitrogen 28), creatinine 113, calcium 40, cholesterol 387.
Understand the difference between urea and urea nitrogen: BUN (mg/dL) multiplied by 2.14 gives urea in mg/dL; the same specimen reported as BUN versus urea differs by that factor. Similarly, ionized versus total calcium, and free versus total hormone concentrations.
Traceability and reference measurement systems matter: results should be traceable to reference materials (e.g., NIST SRM, WHO international standards). Enzymatic activity is reported in U/L (μmol/min/L), not mass concentration. Osmolality (mOsm/kg) versus osmolarity (mOsm/L) differ slightly because of plasma water content.
Rounding and significant figures are tested: report to the precision the method supports, and understand that reference intervals are method-dependent. Converting creatinine from mg/dL to μmol/L uses a factor of 88.4; glucose mg/dL to mmol/L uses 0.0555.
Worked example: a calcium of 10 mg/dL equals 10/40 = 0.25 mmol/L = 2.5 mmol/L (multiply by 10 for the 100 mL denominator). A glucose of 90 mg/dL is 90 × 0.0555 = 5.0 mmol/L.
Trap: mixing BUN and urea, or forgetting the 10× factor when converting mg/dL to mmol/L.