Combined total error in calculated clinical chemistry tests: component analytical error is magnified and decision threshold gray zones widen.

INTRODUCTION: Calculated tests propagate component analytical errors, yet laboratories monitor only the measured components, rarely the calculated quantity. METHODS: Using analytical performance specifications (APSs) from the European Federation of Clinical Chemistry and Laboratory Medicine Biological Variation Database, we propagated bias and imprecision through 5 calculated tests (low-density lipoprotein cholesterol [LDL-C], anion gap, albumin-corrected calcium, the Homeostatic Model Assessment of Insulin Resistance [HOMA-IR], and the De Ritis ratio), representing difference, linear correction, product, and ratio structures. Difference-based and linear-based tests were assessed across clinical scenarios; the ratio was validated by Monte Carlo simulation. RESULTS: At desirable component APSs, the combined total error was 2.1% to 2.8% for albumin-corrected calcium, 25.9% for the De Ritis ratio, and 34% for the HOMA-IR. At typical values, combined errors for LDL-C and the anion gap were 18.5% and 20.8%, respectively, exceeding their respective allowable total error targets (12.0% and 11.3%). Error was concentration dependent: the anion-gap error decreased to 7.4% at 28 mmol/L but increased to 43.0% at 6 mmol/L; the LDL-C error reached 28.6% at 60 mg/dL. The allowable total error-based estimate closely matched the quadrature-bias estimate but underestimated worst-case error. DISCUSSION: Acceptable component performance does not ensure acceptable performance of calculated tests. Derived quantities require specific APSs, and results near clinical decision limits should be interpreted within concentration-dependent analytical gray zones.