What does a wet preparation (wet‑prep) of a vaginal or urethral sample test for?

What Does a Wet Prep Test For?

A wet preparation (wet prep) tests for the three most common causes of vaginal infections—bacterial vaginosis, trichomoniasis, and vulvovaginal candidiasis—by allowing direct microscopic visualization of clue cells, motile trichomonads, and yeast forms, while also facilitating pH measurement and the whiff test. 1, 2

Primary Diagnostic Targets

Bacterial Vaginosis

• Clue cells (epithelial cells covered with bacteria) are the hallmark finding and represent one of the four Amsel criteria needed for diagnosis. 1, 2
• The wet prep enables immediate visualization of these bacterial-covered epithelial cells in the saline specimen. 1, 3
• When combined with vaginal pH >4.5 and a positive whiff test (fishy odor when secretions are mixed with 10% KOH), bacterial vaginosis can be diagnosed clinically without requiring more expensive laboratory testing. 2, 3

Trichomoniasis

• Moving flagellated trichomonads can be directly visualized on wet mount microscopy. 1, 2
• However, this is a critical limitation: wet prep sensitivity for trichomoniasis is only 40-70%, meaning it misses more than half of infections. 2
• The specimen must be examined within 30 minutes to 2 hours, as motile trichomonads lose viability rapidly. 2
• NAATs are strongly preferred for trichomoniasis screening due to superior sensitivity, and a negative wet prep does not rule out infection. 2

Vulvovaginal Candidiasis

• Pseudohyphae and budding yeast become visible on microscopy, particularly when enhanced with 10% KOH preparation. 1, 2
• The KOH dissolves cellular debris while preserving fungal elements, making yeast easier to identify. 1, 2
• When wet mount is negative but symptoms persist, vaginal culture should be ordered to detect non-albicans Candida species that may require alternative therapy. 2

Additional Diagnostic Information

White Blood Cell Count

• An increased number of white blood cells indicates infection or inflammation. 1
• Greater than 10 white blood cells per high-power field suggests increased risk of trichomoniasis, even when trichomonads are not visualized. 4

pH Testing

• Normal vaginal pH is <4.5, suggesting candidiasis or normal flora. 1, 2
• pH >4.5 indicates bacterial vaginosis or trichomoniasis, helping differentiate between conditions. 1, 2, 3
• pH is measured by dipping pH paper in vaginal secretions and reading the color change within 10 seconds. 1

Whiff Test (KOH Preparation)

• Adding 10% KOH to vaginal secretions produces a characteristic fishy amine odor in bacterial vaginosis. 1, 2, 3
• This dual-purpose test also enhances visualization of fungal elements. 1, 2

Critical Limitations and Pitfalls

Poor Sensitivity for Trichomoniasis

• The wet prep misses trichomoniasis 30-50% of the time, making it an unreliable standalone test. 1, 2
• Culture, nucleic acid probe, or NAAT testing are more sensitive and should be ordered when trichomoniasis is suspected. 1, 2

Time-Sensitive Examination

• The wet mount should be read immediately after the pelvic examination to increase the likelihood of detecting trichomonads. 1, 2
• Specimens lose diagnostic value after 30 minutes to 2 hours. 2

Operator-Dependent Accuracy

• Proficiency in microscopic examination is essential, as wide variation exists in interpretation between different laboratories and providers. 2
• Mixed infections may present atypically, requiring clinical judgment. 2

Specimen Collection Errors

• Do not contaminate the specimen with cervical mucus, as this affects test accuracy. 2
• Collect vaginal secretions from pooled discharge or lateral vaginal walls using a cotton swab. 1

When Wet Prep Is Insufficient

Persistent Symptoms with Negative Results

• Order comprehensive NAAT testing for Trichomonas vaginalis, Chlamydia trachomatis, and Neisseria gonorrhoeae simultaneously. 2, 5
• Consider multiplex NAAT panels (e.g., BD Max Vaginal Panel) that detect microbiome-based bacterial vaginosis, Candida species, and Trichomonas with superior sensitivity. 2
• Order vaginal yeast culture when wet mount is negative but symptoms persist, particularly for recurrent infections. 2, 5

Gold Standard Alternatives

• Quantitative Gram stain (Nugent criteria) is the gold standard for bacterial vaginosis but is more specific than necessary for routine clinical diagnosis. 2, 3
• Do not culture G. vaginalis for bacterial vaginosis diagnosis, as it lacks specificity—the organism is present in normal flora. 2, 3

Concurrent STI Testing

• Do not forget STI testing, as many infections coexist with vaginitis, requiring concurrent testing for chlamydia, gonorrhea, and trichomonas. 2, 5