Measurement of urinary free cortisol using the Acs:180 serum cortisol chemiluminescent immunoassay

Measurement of urinary free cortisol using the Bayer Automated Chemiluminescent System (ACS:180 PLUS) was evaluated and compared with an in-house extraction radioimmunoassay (RIA). Inter-assay coefficients of variation were acceptable, being respectively 5.3%, 4.8% and 3.8% at 141, 406 and 942 nmol/L (n=20) for the ACS direct assay and 19.4% and 12.1% at 27 and 116 nmol/L with dichloromethane extraction (n=10). Using tritiated cortisol, mean extraction efficiency for the ACS extraction protocol was 89% compared with 104% for the RIA method (n=6, P<0.001) and using urine spiked with ACS serum calibrator, extraction efficiency was only 76% (standard deviation 8.8%, n=49) on the ACS. Urine spiked with cortisol dissolved in ethanol also gave significantly lower recoveries on the ACS for both direct and extraction methods compared with the RIA. Regression analysis of results from a mixture of control serum samples and samples from patients (n=93) showed good correlations between the direct and extraction ACS methods and the RIA extraction assay. The median concentration in 23 normal subjects and 95% reference intervals (nmol/24 h) were: ACS direct, 237 (135-505); ACS extraction, 91 (33-239); and RIA extraction, 146 (80-334). 5Beta-dihydrocortisol was the only major interferent in all assays. The ACS extraction assay showed acceptable performance and correlated well with the extraction RIA, although there was evidence of matrix-dependent effects causing low recovery.