What does a wet prep (wet preparation) include?

What a Wet Prep Includes

A wet preparation (wet mount) is a microscopic examination of vaginal secretions mixed with normal saline and 10% potassium hydroxide (KOH) to identify infectious organisms and cellular elements, specifically evaluating for trichomonads, clue cells, white blood cells, yeast/hyphae, and performing a whiff test. 1, 2

Components of the Wet Prep Examination

Saline Wet Mount

• Vaginal secretions are collected with a cotton swab and mixed with 0.5-1 mL of normal saline on a microscope slide 2, 3
• The preparation is examined under low- and high-dry power microscopy (typically 40× and 60×) 2, 4
• Specific elements evaluated include:
  • Trichomonads: Moving, flagellated organisms indicating Trichomonas vaginalis infection 1
  • Clue cells: Epithelial cells covered with bacteria, diagnostic for bacterial vaginosis 1, 2
  • White blood cells (WBCs): Quantified as none, few, moderate, or many; elevated numbers suggest infection 5, 6
  • Lactobacilli: Rod-shaped bacteria that are normal vaginal flora 4
  • Squamous epithelial cells: Assessed for maturation status 4

KOH Preparation

• A second slide is prepared by mixing vaginal secretions with 10% potassium hydroxide 1
• The KOH disrupts cellular material to improve visualization of fungal elements 1
• This preparation is examined for:
  • Yeast cells and pseudohyphae/hyphae: Indicating Candida species infection 1
  • Whiff test: The characteristic fishy amine odor produced when KOH is added to secretions, positive in bacterial vaginosis 1, 2

pH Testing

• Vaginal pH is measured using narrow-range pH paper applied to vaginal secretions 2, 3
• Normal vaginal pH is <4.5 1, 2
• Elevated pH (>4.5) suggests bacterial vaginosis or trichomoniasis 2, 3

Clinical Application

Diagnostic Criteria Using Wet Prep

• For bacterial vaginosis (Amsel criteria), three of four findings are required: homogeneous white discharge, pH >4.5, positive whiff test, and clue cells on microscopy 2
• For vulvovaginal candidiasis: Wet prep demonstrates yeast or pseudohyphae with normal pH (<4.5) 1
• For trichomoniasis: Motile trichomonads are identified on saline wet mount, though sensitivity is only 60-70% 1, 7

Important Limitations

• Wet mount microscopy misses trichomoniasis in 30-50% of cases; more sensitive methods include culture or nucleic acid amplification tests 5
• The sensitivity of clinic wet prep for T. vaginalis is approximately 68%, meaning 30% of cases may be missed without additional testing 8
• Vaginal pool specimens are more sensitive than endocervical specimens for detecting trichomonads (100% vs 50%), pseudohyphae (76% vs 65%), and clue cells (95% vs 50%) 9

Timing Considerations

• Wet mount examination should be performed within 30 minutes of collection for optimal results, with a maximum of 2 hours acceptable 3
• Immediate viewing is required for detecting motile trichomonads 1

Common Pitfalls to Avoid

• Do not use Papanicolaou (Pap) test to diagnose T. vaginalis due to poor sensitivity and specificity 1
• Avoid contaminating the swab with cervical mucus, as this may affect test results 3
• The presence of Candida on culture without symptoms should not lead to treatment, as 10-20% of women harbor yeast normally 1
• Moderate or many white blood cells on wet prep increase the probability of N. gonorrhoeae or C. trachomatis infection (OR 1.58 and 2.47, respectively), but clue cells and trichomonads do not correlate with these infections 6