A wet-mount preparation is performed by combining vaginal discharge and one or two drops of normal saline on a microscope slide. Several microscope fields should be examined for motile trichomonads and for the clue cells of bacterial vaginosis. The sensitivity of these tests is about 60% for trichomoniasis and bacterial vaginosis, but the specificity is 98%. Other possible findings on the wet-mount examination include fungal hyphae (candidiasis), numerous white blood cells (trichomoniasis), and parabasal cells (atrophic vaginitis).
Potassium Hydroxide (KOH) Preparation
To perform a “KOH prep,” a sample of vaginal discharge is combined with one or two drops of 10% to 20% potassium hydroxide (KOH) on a microscope slide. The whiff test is performed at this point by smelling for a fishy amine odor, which occurs in bacterial vaginosis. Once the slide has been dried by flame or air, it is examined under low power for candidal hyphae, spores, and mycelia. Scanning for these fungal elements is about 60% sensitive for candidiasis.
The pH level of vaginal secretions is measured by placing litmus paper in pooled secretions or against the vaginal wall. Normal vaginal pH is 3.8 to 4.2, whereas in bacterial vaginosis, the pH is above 4.5 in about 90% of cases. The vaginal pH can also be elevated in trichomoniasis and atrophic vaginitis, but to a lesser degree.
Cultures are not typically done to diagnose vaginitis but can be helpful in cases in which the diagnosis is in question or when treatment has failed. Cultures for T. vaginalis on Diamond’s medium and DNA probes have sensitivities of 90% and 95% respectively.
Cultures can also be useful in cases of candidiasis but many women without vulvovaginal candidiasis are colonized with Candidal. Many clinicians will treat presumptively for yeast if vaginal pH is normal and non-diagnostic organisms are found on microscopy.
Cultures for G. vaginalis are not helpful for diagnosis, since they are positive in 50% of women without symptoms. Diagnostic cards (Femcard) have recently been introduced that can detect elevated pH and amines in vaginal fluid. These cards have a sensitivity of 87%, and a specificity of 92%.
In general, Amsel’s criteria can be useful, and the presence of three of these criteria is 90% sensitive for bacterial vaginosis:
• Thin, homogeneous discharge
• Positive whiff test
• Clue cells
• Vaginal pH greater than 4.5