Wet Mount

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Sensitivity of Wet Mount vs. Culture,
Pap Smear, and PCR3

Comparison of sensitivity of four diagnostic tests for trichomoniasis. Wet mount is 52 percent, culture is 78%, pap smear is 24 percent, and PCR is 84 percent. The sample size is 337 women.

Diagnosis in clinical practice is usually based on identification of motile trichomonads in a saline wet mount preparation of vaginal discharge. The wet mount is a quick, easy, inexpensive means for identifying trichomoniasis.

Unfortunately, multiple studies have shown that compared to culture, wet mount sensitivity is only 45%-60%.1-3 A 2002 study comparing trichomoniasis diagnostic methods found wet mount to be 52% sensitive, compared to 78% for culture and 84% for polymerase chain reaction.3

There are many confounding variables that can affect the results obtained from a wet mount. The first is the number of motile trichomonads and white blood cells (WBCs) present. WBCs and trichomonads are similar in size and shape so only motile organisms should be identified as T. vaginalis.4

The clinician's skill level in handling wet mount samples may affect the accuracy of diagnosis.

Decreasing Shelf-life of Trichomonas vaginalis
on Wet Mount Preparations5

Bar graph depicting the survival rate of Trichomoniasis vaginalis on wet mount preparations over time. At 0 minutes, the survival rate is 100 percent, at 10 minutes it is 80 percent, at 30 minutes it is 65 percent, and at 120 minutes it is 22 percent.

Another factor is the time interval between specimen collection and microscopic examination. A 2003 study of the amount of time trichomonads remained motile on a wet mount found that 10 minutes after the initial reading, there were no motile organisms in 20% of the wet preparations initially positive for T. vaginalis. After two hours, 78% of the samples that were initially positive had become negative. With this in mind, wet mount preps should be examined immediately after they are collected.5

  1. Lossick JG, Kent HL. Trichomoniasis: trends in diagnosis and management. Am J Obstet Gynecol. 1991 Oct;165(4 Pt 2):1217-22.
  2. Lobo TT, Feijó G, Carvalho SE, Costa PL, Chagas C, Xavier J, Simoes-Barbosa A. A comparative evaluation of the Papanicolaou test for the diagnosis of trichomoniasis. Sex Transm Dis. 2003 Sep;30(9):694-9.
  3. Wendel KA, Erbelding EJ, Gaydos CA, Rompalo AM. Trichomonas vaginalis polymerase chain reaction compared with standard diagnostic and therapeutic protocols for detection and treatment of vaginal trichomoniasis. Clin Infect Dis. 2002 Sep 1;35(5):576-80. Epub 2002 Aug 6.
  4. Thomason JL, Gelbart SM. Trichomonas vaginalis. Obstet Gynecol. 1989 Sep;74(3 Pt 2):536-41.
  5. Kingston MA, Bansal D, Carlin EM. 'Shelf life' of Trichomonas vaginalis. Int J STD AIDS. 2003 Jan;14(1):28-9.