Quantification of Chlamydia trachomatis in cervical and urine specimens from women attending a genitourinary medicine clinic: implications for screening strategies

Population screening and intervention programmes can reduce the prevalence and incidence of infection with Chlamydia trachomatis , especially if sensitive molecular diagnostic tests are used. However, diagnostic tests that perform well on genitourinary medicine GUM clinic populations may be less useful for screening, particularly if the majority of infected subjects are asymptomatic and their samples contain fewer organisms. We have compared the extent of low organism load in cervical and urine samples from symptomatic and asymptomatic chlamydia positive women, by using a direct fluorescent antibody staining method and counting the chlamydial elementary bodies EBs . We have investigated the ability of an enzyme immunoassay EIA; MicroTrak and a DNA amplification ligase chain reaction; LCR assay to detect low numbers of organisms in cervical samples and the ability of the LCR assay to detect low numbers of organisms in urine. A low organism load 10 EBs was seen by direct fluorescent antibody DFA staining in about 30 of cervical samples and in about 75 of urines from chlamydia positive women; the proportions in symptomatic women were not significantly different from those in asymptomatic women. The EIA identified only 16 of cervical samples that contained 10 EBs by DFA staining; the LCR identified 100 of cervical samples and 93 of urine samples that contained 10 EBs by DFA staining. The findings suggest that the ability of chlamydial diagnostic tests to identify positive women should be similar among patients attending a GUM clinic and those taking part in a population screening programme, and that sensitive molecular assays such as the LCR should identify subjects with a low organism load in both groups.