Qualitative and quantitative aspects of the ligase chain reaction assay for Chlamydia trachomatis in genital tract samples and urines

The performance of the ligase chain reaction (LCR) assay for Chlamydia trac homatis was evaluated in a genitourinary medicine (GUM) clinic population. Its sensitivity was 100%, 91% and 95%, respectively, for cervical, vaginal and urine samples from 417 women, when compared with direct fluorescent ant ibody (DFA) staining of cervical samples, and 100% and 91%, respectively, f or urethral and urine samples from 317 men, when compared with DFA staining of urethral smears. An enzyme immunoassay (ETA) was only 65% sensitive for cervical samples. Urethral swabs from a number of treated men remained LCR -positive when antigen was no longer detectable by DFA staining. An associa tion between quantitative data from the LCR assay (i.e. the optical density of samples, measured in relation to internal controls and calibrators) and the antigen load of the samples, measured by DFA staining, indicated a lac k of significant inhibition in the LCR assay in this study. This was probab ly due to freezing of the samples before testing. Diluting 20 LCR-positive urines with a range of antigen loads resulted in loss of positivity in 3, a nd a reduction in the signal in 13. The implications of the antigen load on the performance of detection assays for chlamydia-positive patients are di scussed.