Use of polymerase chain reaction on pooled cervical swabs to detect Chlamydia trachomatis infections in female sex workers in Singapore

HH Tan, R Chan
Correspondence: Dr Tan Hiok Hee, hhtan@nsc.gov.sg

ABSTRACT
Introduction Infections caused by Chlamydia trachomatis (C. trachomatis) is one of the commonest sexually transmitted infections (STI). As there is currently no laboratory in Singapore that offers the chlamydia culture tests, alternative laboratory methods were developed and antigen detection methods such as enzyme immunoassays (EIA) proved to be even less sensitive than cell culture. This study was done to assess the accuracy and sensitivity of pooling five endocervical swabs collected for C. trachomatis testing by polymerase chain reaction (PCR) technology, as compared to the currently used EIA on individual swabs, in female sex workers who were seen at a STI clinic in Singapore.
Methods A total of 1,182 endocervical swab specimens were analysed by EIA as well as in pools of five specimens using PCR. Any pool with a positive PCR result for C. trachomatis infections was subjected to repeat PCR testing of the five individual specimens in the pool.
Results There were a total of 48 confirmed cases of C. trachomatis infection. EIA detected 19 positive samples for C. trachomatis, yielding a prevalence of 1.6 percent among the sex workers tested. Pooled PCR testing showed a higher prevalence rate of 4.1 percent, with 48 PCR positive samples. All cases that were EIA positive were also PCR positive. Individual runs on 200 random samples as well as on 220 individual samples from positive pooled results showed PCR inhibition rates ranging from 1.5 percent to 2.3 percent. However, the PCR inhibition rate was 0 percent with the use of pooling. Sensitivity of EIA was 39.6 percent, with 100 percent specificity. EIA tests had a false negative rate of 60.4 percent. PCR was found to be 100 percent sensitive and specific.
Conclusion C. trachomatis infections among female sex workers attending the clinic were found to be higher using PCR technology. Less sensitive methods such as EIA result in undertreatment of otherwise undetected cases. The pooling strategy using pool sizes of five specimens, with a disease prevalence of 4.1 percent is reliable and cost-effective, and has since been introduced in the current medical surveillance scheme for sex workers in the clinic.

Keywords: Chlamydia trachomatis, enzyme immunoassays, polymerase chain reaction
Singapore Med J 2005; 46(5): 215-218