Literature review > Issue_1 > Review Gaydos et al. 

The advent of nucleic acid amplification tests (NAATs) has opened the door to innovative approaches to screening for C. trachomatis and N. gonorrhoeae infections. The sensitivity of NAATs is much greater than older methods permitting the use of nontraditional specimens such as urine and vaginal swabs for the detection of these STD pathogens. Several investigators recently have taken advantage of NAAT superior performance characteristics to explore the possibility of women testing themselves at home and mailing the specimens directly to the laboratory. Thus far, most of this work has utilized self-collection of urine specimens. However, because of the potential for spillage, urine specimens present potential problems with shipment through the mail. Recent work has suggested that vaginal swab specimens are as sensitive as endocervical swabs and perhaps more sensitive than urine specimens for the detection of chlamydial and gonococcal infections. Vaginal swabs have a clear advantage over urine specimens for self testing at home if it can be shown that the specimen will withstand drying for the period of time required for shipment to the laboratory. In this paper, Gaydos et al. test the hypothesis that dry vaginal swabs will perform in a PCR assay as well as swabs transported in a liquid medium.

The study population consisted of 793 active duty women 18 to 59 years of age who presented to a military clinic in North Carolina for the evaluation of genitourinary symptoms (83% of the participants) or as a contact of a male with a diagnosed STD or for routine STD screening. An endocervical swab was collected and tested locally by the Syva enzyme immunoassay (EIA) for chlamydia. Three vaginal swabs were obtained. One was placed in Roche Amplicor CT/NG PCR transport media, one was placed in a dry tube and the third was placed in liquid transport media for the ligase chain reaction test. The latter specimen along with extra endocervical specimens were used for adjudicating discrepant results between the Syva EIA test for chlamydia and the vaginal swab PCR results. The specimens were mailed from the clinic in North Carolina to the laboratory in Baltimore where the NAATs were run. N. gonorrhoeae was detected by culture and discrepant results between culture and the two vaginal swabs were resolved using alternate gene target PCR assays run with the two extra endocervical swabs.

After adjudication of discrepant results, 92 patients were identified as being C. trachomatis infected. Twenty-seven women were infected by N. gonorrhoeae. In no case did the results for the dry vaginal swab differ significantly from those for the wet vaginal swab. The important finding in this study was that sensitivity of the PCR assay for both organisms was not affected by transporting the specimens dry. If these data are supported by further studies it is likely that vaginal swabs shipped in a dry state will become the basis for future studies of self testing at home as a means of screening women for chlamydial and gonococcal infections.

There are a several problems with this study that should be borne in mind. Firstly, the specimens were shipped at 4°C. Maintenance of a cold chain would greatly complicate the development of self-testing screening strategies. It will be necessary to demonstrate in future studies that dry specimens are stable at normal shipping temperatures. Generally bacterial DNA is stable over long periods at room temperature so it is very likely that this will be the case for chlamydial and gonococcal DNA. Secondly, apparently the authors used the original optical density cut off values for a positive PCR result set by the manufacturer for the N gonorrhoeae research assay. These early studies found a relatively high false positive rate for the gonococcus, as was observed here by Gaydos et al. Subsequently, it was found that the Amplicor N. gonorrhoeae PCR target cross reacts with saprophytic Neisseria species, resulting in a relatively high number of false positive tests. The optical density signal produced by these cross-reacting DNA targets are relatively weak so, by raising the cut-off value, most of the false positives are eliminated. On the other hand, changing these criteria may result in a decrease in sensitivity of the assay so the data presented in this study bear repeating before any firm conclusions can be reached concerning the utility of the system for the detection of N. gonorrhoeae. Finally, it should be pointed out that the sensitivity of culture for N. gonorrhoeae in this study is lower than reported by other investigators using NAAT's as the gold standard. The gonococcus is a fastidious organism and careful attention to details such as media freshness and transport conditions are required to achieve optimal results. When done correctly, gonococcal culture of the endocervix has proven to be a very sensitive test. The main advantage of NAATs over culture is the ability to test for the organism using urine and vaginal swab specimens, not increased sensitivity.

Over the next few years we can expect to see many new studies published on the subject of self testing by women for C trachomatis and N. gonorrhoeae using both with urine and vaginal swab specimens. The results of these studies will be greeted with considerable interest because of the great potential such approaches have for controlling the spread of C. trachomatis and N. gonorrhoeae infections.

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