How should I perform a wet mount microscopy of vaginal discharge?

How to Perform a Wet Mount of Vaginal Discharge

To perform a wet mount microscopy of vaginal discharge, collect a sample of vaginal discharge with a cotton swab, dilute it in 1-2 drops of 0.9% normal saline on one slide and 10% potassium hydroxide (KOH) on a second slide, apply coverslips, and examine under a microscope at low- and high-dry power. 1

Sample Collection

• Use a cotton swab to collect vaginal secretions from the pooled vaginal discharge during speculum examination 1
• The lower third of the vagina has the highest sensitivity for detecting Candida (60%) and bacterial vaginosis (92%), while the anterior fornix performs better for cytolytic vaginosis (75% sensitivity) 2
• Consider collecting two samples (anterior fornix and lower third) to overcome variation in results according to sampling site 2
• Samples can be placed in a tube with 1 mL of normal saline solution for transport to prepare slides, or the slide can be directly smeared with the swab 1

Slide Preparation

Saline Wet Mount (Primary Slide)

• Place the vaginal discharge sample in 1-2 drops of 0.9% normal saline solution on a glass slide 1
• Apply a coverslip 1
• This preparation is used to identify motile Trichomonas vaginalis, clue cells of bacterial vaginosis, white blood cells, and lactobacilli 1

KOH Preparation (Second Slide)

• Place a separate sample in 1-2 drops of 10% potassium hydroxide (KOH) solution on a second glass slide 1
• Immediately check for an amine ("fishy") odor after applying KOH—this "whiff test" suggests bacterial vaginosis or trichomoniasis 1
• Apply a coverslip 1
• This preparation is used to identify yeast or pseudohyphae of Candida species, which are more easily visualized after KOH dissolves cellular debris 1

Microscopic Examination

• Examine slides immediately under a microscope at low- and high-dry power 1
• The wet mount should ideally be read as soon as the pelvic examination is completed to increase the likelihood of detecting motile trichomonads 1
• Phase-contrast microscopy is recommended to improve performance and reproducibility 3

What to Look For on Saline Wet Mount:

• Motile Trichomonas vaginalis (flagellated protozoa) 1
• Clue cells (epithelial cells covered with bacteria) indicating bacterial vaginosis 1
• Increased white blood cells as an indication of infection 1
• Lactobacilli (normal flora) 1

What to Look For on KOH Preparation:

• Yeast or pseudohyphae of Candida species 1
• Budding yeast forms 1

Additional Testing During Wet Mount Evaluation

• Measure vaginal pH using narrow-range pH paper—normal pH is **<4.5**; pH >4.5 suggests bacterial vaginosis or trichomoniasis 1
• An easy method is to dip pH paper in secretions left on the speculum tip after removal and read within 10 seconds 1
• Perform the whiff test by noting any fishy odor when KOH is applied, which is positive in bacterial vaginosis due to amines in the discharge 1

Important Caveats

• Wet mount has variable sensitivity: 40-80% for trichomoniasis and limited sensitivity for candidiasis compared to culture or NAAT 1, 3
• The wet mount can miss trichomoniasis 30-50% of the time; culture, nucleic acid probe, or antigen-based rapid testing may be more sensitive 1
• Culture is more sensitive than microscopic examination for T. vaginalis or Candida species, but should be reserved for recurrent/complicated cases or when microscopy is negative 1
• Not all Candida species form pseudohyphae (e.g., C. glabrata shows only yeast cells), which can lead to false-negative microscopy 1
• Transport time matters: samples submitted in saline or transport swab should be examined within 2 hours at room temperature for optimal results 1
• With practice, wet mount microscopy can be mastered—10 hours of hands-on training enables fair to good concordance with expert readings 4