In this study, we found that the prevalence of T. vaginalis
was high among adolescents who were wet mount negative (8.8%). Risk factors did not help to identify wet-mount-negative adolescent women who tested positive for T. vaginalis
. This is in contrast to the findings reported by Swygard et al., who recommended performing T. vaginalis
culture for wet-mount-negative women with one of three risk factors: black race, reported T. vaginalis
contact, or any drug use (13
). Adding culture to the evaluation of women with any of these three factors resulted in the detection of 97.3% of T. vaginalis
infections. That study enrolled 2,194 adult women from an STD clinic setting where both a wet mount and T. vaginalis
culture were routine. In contrast, in our study, neither race nor a history of prior T. vaginalis
infection was a predictor of T. vaginalis
infection in women who were wet mount negative. We did not assess participants for recent T. vaginalis
contact or drug use. In both our study and that of Swygard et al., neither clinical laboratory data (such as the presence of WBCs or clue cells) nor patient-reported symptoms were reliably associated with a T. vaginalis
diagnosis. In addition, we included more-sensitive test methods (the rapid test and NAAT) to establish our definition of a true-positive infection. This could result in a decrease in the sensitivity of culture in our study or an increase in the sensitivity of culture in the study of Swygard et al.
We also found that a delay of more than 50 min between specimen collection and testing was associated with an increased likelihood of a positive secondary rapid test. This may be explained by the difference in the test mechanism between the rapid test and culture. Culture requires live trichomonads, whereas the rapid test requires that the trichomonad be dead and its membrane disrupted. A lengthy stay in saline increases the likelihood of trichomonad death, releasing membrane proteins and thus decreasing the sensitivity of culture and increasing the sensitivity of the rapid test. In addition, other, unmeasured clinical factors may have contributed to this result. While we do not recommend delaying the reading for a point-of-care test, the finding that sensitivity does not decrease with time is reassuring in usual-care settings where a rapid test might be delayed until the wet-mount results are known.
Although we found that secondary culture of used wet-mount saline is the least sensitive method after a wet mount only, others have reported that delayed inoculation of a T. vaginalis
culture pouch using a primary swab yields good results. Schwebke et al. studied 150 women using a wet mount, primary culture (inoculated at the bedside), and a delayed culture (a primary swab held until after the wet mount was completed). For 39 T. vaginalis
culture-positive subjects, delayed culture was 100% sensitive compared to primary culture, regardless of whether the delayed swab was held in saline or held dry (11
). The better results with a primary swab may be related to the increased organism load found on a primary swab relative to that in the residual wet-mount saline that we used for the secondary culture.
Because T. vaginalis infection is linked to serious health outcomes, such as the acquisition and shedding of HIV, our findings suggest that in similar settings, all adolescent women may benefit from additional T. vaginalis testing regardless of other risk factors. In the hands of experienced providers, wet-mount tests yield valuable information on other parameters, such as the presence of WBCs, clue cells, or yeast. Therefore, despite its moderate sensitivity for T. vaginalis, the wet mount is a reasonable first step in the evaluation of women at risk for T. vaginalis and other STIs. Point-of-care tests, such as wet-mount and rapid tests, are important STI diagnostic strategies, since they allow immediate counseling and treatment. Therefore, we propose using a stepwise diagnostic strategy, as follows. (i) Since the wet mount is a useful and inexpensive test, all women at risk for STIs should have a wet mount performed. When the wet-mount swab is obtained, an additional primary vaginal swab can be obtained and held either in saline or as a dry swab. (ii) If the wet mount is negative for trichomonads, a rapid antigen test can be performed using the saved primary swab or the used wet-mount swab with no loss of sensitivity. (iii) If both the wet-mount and rapid antigen tests are negative, the provider should consider inoculating a T. vaginalis culture using the saved primary swab if T. vaginalis is highly suspected or if the prevalence of T. vaginalis is high in the population. Because the NAAT for T. vaginalis is not yet commercially available, we cannot recommend its use as a primary or secondary screening method for T. vaginalis infections.
A limitation to this study is that it may not be generalized to other populations because of the high prevalence of T. vaginalis and of risk behaviors (i.e., multiple partners) associated with T. vaginalis infection in our sample. Also, we did not calculate the costs associated with the use of additional T. vaginalis tests. It is reassuring that the manufacturers' listed costs for culture and the rapid test are significantly less than those for other commonly used STI tests, such as NAATs for Chlamydia spp. The incremental cost, that is, the cost per additional T. vaginalis infection detected, will vary depending on the prevalence of T. vaginalis in the population tested. At present, we still do not have sufficient data on health outcomes related to T. vaginalis infection upon which to base estimates of the cost-effectiveness of T. vaginalis screening.
The main outcome of this study is that our findings support and expand upon the CDC's recommendation to provide additional T. vaginalis testing for wet-mount-negative women. First, when a wet mount is routinely performed, a rapid test for T. vaginalis can be delayed until after the wet mount is read, and it can be performed on the used wet mount swab with no loss of sensitivity. A stepwise approach using an additional T. vaginalis test for wet-mount-negative women will increase the rate of detection of T. vaginalis and is recommended for all adolescent women. Second, we emphasize that it is important from a public health perspective to screen asymptomatic adolescent women for T. vaginalis. Further study is needed to assess the degree to which the increased cost of additional T. vaginalis testing may be balanced by public health benefits.