Literature review > Issue 8 > Review on Sturm et al. 

Since a large proportion of the individuals with sexually transmitted infections (STI) are asymptomatic and therefore unlikely to seek medical help, control programmes targeting only symptomatic individuals have made little impact on the prevalence of these infections. It becomes therefore necessary to identify infected asymptomatic individuals. This requires not only sensitive and specific tests but also reliable methods acceptable to individuals and care givers for specimen collection and transportation. Performance of self collected samples in community settings therefore needs to be evaluated to understand the suitability of such tests in intervention programmes.

This study done at a primary health clinic in rural South Africa on antenatal women, has evaluated the performance of vaginal tampons as a method of collecting samples for the diagnosis of non ulcerative STIs due to Chlamydia trachomatis, Neisseria gonorrhoeae and Trichomonas vaginalis using nucleic acid amplification tests (NAAT). For C trachomatis and N. gonorrhoeae, strand displacement amplification assay (BDProbeTec^TM assay) was performed and for T. vaginalis a PCR was carried out using SAR1 and SAR 2 as primers.

Defining a reference standard for comparing tests is problematic in STI diagnostics since the 'gold standard' culture has much less sensitivity than NAAT. NAAT on the other hand is more prone to false positive results. Because of these reasons many studies utilise a discrepancy analysis between tests under evaluation. Testing multiple sites and using multiple methods increase the sensitivity in accurately classifying infected individuals [1]. In this report, to identify women with true infection, samples were collected from multiple sites - endocervix/vagina and urine and tested by more than one method -culture (except for C. trachomatis) and NAAT on three consecutive days. Any individual with a positive culture or more than one specimen positive by NAAT, or a single specimen positive but confirmed by repeat testing on the same sample, was considered infected. By these criteria, 77 (42%) of the 185 women included were infected with at least one organism under study. T. vaginalis infection was present in 37%, C. trachomatis in 14% and N. gonorrhoeae in 8%.

Performance characteristics of individual methods were assessed using data obtained on the first visit only. The findings therefore are relevant to most settings, where women get tested only once. All tests had excellent specificity (99 - 100%) and predictive values. For detecting T. vaginalis, sensitivity (94%) of PCR on the tampon specimen was significantly higher than that of culture (50%) or PCR on urine (53%). For N. gonorrhoeae, NAAT on tampon specimen had the highest sensitivity (79%) while NAAT on urine had unacceptably low sensitivity (43%). However, the confidence intervals were very wide and the differences not statistically significant. The positive predictive value for NAAT on tampons was only 85%. For C. trachomatis detection, all methods performed equally with sensitivity rates of 84 - 92%. Overall, only 64 (60%) of the total 107 pathogens identified could be detected by conventional methods. By NAAT, 87(81%) of these 107 could be identified on three tampon specimens.

Equivocal and indeterminate results necessitating repeat testing with NAAT were obtained more often with tampon specimens. Inhibition rate for T vaginalis PCR was found to be 6%, in an experiment where tampons were spiked with T vaginalis DNA and then tested using NAAT.

Self insertion of vaginal tampons was acceptable in 137 (87%) of the 158 assessed. Only 47% of the infected had abnormal discharge. The study also found that a proportion of women with symptoms were also unlikely to seek medical help.

This study confirms that tampon samples are suitable collection devices in community settings in South Africa for NAAT used for the diagnosis of non-ulcerative STIs. It also shows once again that NAAT are superior to culture in terms of sensitivity and that specificity and predictive values are similar to that of culture in communities with higher prevalence of STIs. More information is, however, required for low prevalence areas. Similarly, acceptability of tampon is likely to differ among populations in different parts of the world.

Although the system is likely to perform well in intervention programmes, the cost is likely to be prohibitive to be used on a routine basis in low income countries with high prevalence rates of STIs. Therefore, when these tests will become available, affordable, and acceptable on large scale population based programmes remain to be seen.

References:

1. Centres for Disease Control and Prevention, Screening tests to detect Chlamydia trachomatis and Neisseria gonorrhoeae infections, Morbidity Mortality Weekly Report 2002; 51: 1 -38

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