Development of an Assay for Urinary Free Cortisol Determination on the Technicon Immuno 1® System

Objectives: To develop and evaluate a method using an ethyl acetate extraction procedure for the determination of urinary free cortisol on the Technicon Immuno 1® system from Bayer Corporation. Design and Methods: We tested the assay precision, linearity, and correlation with the Urinary Kallestad™ Quanticoat Cortisol radioimmunoassay. We also studied the efficiency of the extraction procedure, performed a cross-reactivity study with different cortisol metabolites, and determined the reference values. Results: The assay shows within-run CVs varying from 1.6 to 5.3% and between-day CVs from 2.7 to 6.1% for urinary free cortisol concentrations from 58 to 1097 nmol/L. The assay demonstrates an excellent linearity and a very good correlation with the Kallestad Quanticoat Cortisol assay (slope = 0.94, y-intercept = 29 nmol/L, Syx = 54 nmol/L, r = 0.996). The reference values were estimated at 42–281 nmol/d. The extraction procedure shows an average recovery of 99.0% and minimal interference with the cortisol metabolites tested with the exception of cortisone. Conclusions: The evaluation shows that the developed assay has the analytical characteristics required for its utilization in a clinical laboratory