What is a wet prep (wet mount) in the context of diagnosing vaginal infections in female patients of reproductive age?

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Last updated: January 22, 2026 • View editorial policy

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What is a Wet Prep?

A wet prep (wet mount) is an immediate point-of-care microscopic examination of vaginal secretions mixed with saline and potassium hydroxide (KOH) that allows direct visualization of the three most common causes of vaginitis—bacterial vaginosis, trichomoniasis, and candidiasis—which together account for approximately 90% of vaginal infection cases. 1

Components of the Test

The wet prep consists of two separate microscopic preparations that serve distinct diagnostic purposes:

Saline Wet Mount

  • Vaginal secretions are mixed with normal saline and examined immediately under microscopy to identify clue cells (indicating bacterial vaginosis), motile flagellated trichomonads (indicating trichomoniasis), and white blood cells 2
  • The specimen must be examined within 30 minutes to 2 hours of collection, as motile trichomonads rapidly lose viability and become undetectable after this window 1, 2

KOH Preparation (10%)

  • A separate specimen is mixed with 10% potassium hydroxide to enhance visualization of pseudohyphae and budding yeast for candidiasis diagnosis 1, 2
  • The KOH preparation also produces the characteristic "fishy" odor when bacterial vaginosis is present, known as the whiff test 1

Additional Testing Performed Simultaneously

pH Testing

  • Normal vaginal pH is <4.5, which suggests candidiasis 1, 2
  • pH >4.5 indicates bacterial vaginosis or trichomoniasis 1, 2

Whiff Test

  • Adding KOH to vaginal secretions releases a fishy amine odor when bacterial vaginosis is present, even when clue cells are not visualized 1

What the Wet Prep Detects

Bacterial Vaginosis

  • Clue cells (vaginal epithelial cells covered with bacteria) are usually easily identified in the saline specimen 1
  • Clue cells represent one of the four Amsel criteria needed for bacterial vaginosis diagnosis 1

Trichomoniasis

  • Motile flagellated trichomonads can be directly visualized swimming in the saline preparation 1, 3
  • However, sensitivity is only 40-70% compared to culture or NAAT, meaning more than half of infections can be missed 1

Vulvovaginal Candidiasis

  • Pseudohyphae and budding yeast become visible, particularly when enhanced with the KOH preparation 1, 3

Critical Limitations and Pitfalls

Time-Sensitive Nature

  • The specimen must be examined within 30 minutes to 2 hours for best results, as motile trichomonads lose motility over time 1, 2
  • This narrow window makes wet prep impractical for many clinical settings without immediate microscopy capability 1

Poor Sensitivity for Trichomoniasis

  • Sensitivity for trichomoniasis is only 40-80%, meaning negative wet prep does not rule out infection 1
  • NAAT testing is preferred for trichomoniasis screening due to superior sensitivity, with recent data showing Trichomonas prevalence equal to or greater than chlamydia and gonorrhea in certain populations 1

Operator-Dependent Interpretation

  • Proficiency in microscopic examination is essential, as infections may be mixed or present atypically 1
  • Wide variation exists in interpretation between different laboratories and providers 1

Specimen Collection Technique

  • Collect vaginal swabs from pooled vaginal discharge or lateral vaginal walls 2, 4
  • Do not contaminate the specimen with cervical mucus, as this affects test accuracy 1, 4

When to Order Additional Testing

NAAT Testing Should Be Ordered Simultaneously

  • NAAT for Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis should be ordered simultaneously with wet prep to ensure comprehensive diagnosis 2
  • Many infections coexist with vaginitis, requiring concurrent STI testing 1

Culture Indications

  • Culture is indicated when wet mount is negative but symptoms persist, or in recurrent infections 1
  • For recurrent yeast infections, culture is mandatory to identify non-albicans Candida species that require different treatment 1, 4

Gram Stain Alternative

  • Gram stain (Nugent criteria) is the gold standard for bacterial vaginosis but is more specific than necessary for routine clinical diagnosis 1
  • Do not culture G. vaginalis for bacterial vaginosis diagnosis, as it lacks specificity 1

References

1
Guideline

Diagnosing Vaginal Infections with Wet Prep

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

2
Guideline

Diagnostic Approach for Vaginal Discharge

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

3
Research

Diagnosis of vaginitis.

American family physician, 2000

4
Guideline

Room Temperature Stability of Liquid Amies Media for Vaginal Pathogen Detection

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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