The evaluation of five commercial serum cortisol kits regarding precision and accuracy.

We evaluated Amerlex (Amersham International), Corti-Shure (Nuclear Medical Laboratories), Gammacoat (Clinical Assays), Coat-A-Count and DPC-direct (Diagnostic Products Corporation) radioimmunoassay kits for determination of cortisol in sera. The between- and within-batch precision (coefficient of variation) ranged between 4.4-12.5% and 3.0-10.9% respectively. No one kit exhibited a clearly better precision throughout the assay range. All kits displayed good sensitivity and parallelism. Cortisol concentrations determined by kit methods, in two assayed control sera, were 3-14% higher than by gas chromatography/mass spectrometry. Coat-A-Count and Gammacoat were technically the simplest assays while Corti-Shure and DPC-direct kits had the best designed standard curves over the diagnostic range. The Amerlex kit had the greatest sensitivity, showed least shift in the dose-response curve between assay and was the only assay that came to equilibrium within the specified incubation time.