- To determine if the time elapsed since previous voiding affects the sensitivity of an enzyme immunoassay (EIA) to detect Chlamydia trachomatis in urine, 882 women and 428 men were tested for chlamydial infection in urethral specimens by isolation in cell culture (women and men) and EIA with blocking confirmation (women only). Of the 36 women (4.1%) and 38 men (8.9%) who were positive for Chlamydia trachomatis in the urethra, 55.5% (20/36) and 81.6% (31/38) respectively were positive in the first void urine (FVU) sediment by confirmed EIA. In women the sensitivity of the EIA performed on FVU was 67.8% (19/28) if the urine had been in the bladder < 3 hours and decreased to 12.5% (1/8) if longer times had elapsed (odds ratio 13.7; 95% confidence interval 1.4 to 700.0) with no obvious confounding. In men a weaker association was present (odds ratio 2.7; 95% confidence interval 0.4 to 22.3). These findings should enable investigators to optimize the analysis of urine to diagnose chlamydial infections.