How are the results of a wet prep (wet preparation) determined?

How Wet Prep Results Are Determined

A wet preparation is interpreted by systematically examining vaginal secretions mixed with saline and potassium hydroxide (KOH) under microscopy to identify specific cellular elements and organisms that diagnose vaginal infections. 1

Specimen Collection and Preparation

• Vaginal secretions are collected with a cotton swab and mixed with 0.5-1 mL of normal saline on a microscope slide 1
• A second slide is prepared by mixing vaginal secretions with 10% KOH to improve visualization of fungal elements 2, 1
• The saline preparation is examined under low- and high-dry power microscopy, typically at 40× and 60× magnification 1

Critical Elements Evaluated on Saline Wet Mount

Trichomonads

• Moving, flagellated organisms indicate Trichomonas vaginalis infection 1
• Motile forms must be identified, requiring examination within 30 minutes to 1 hour of collection for optimal detection 3
• Trichomonad motility is 100% at 30 minutes but decreases by 3-15% each subsequent hour when stored in saline 3
• Wet mount sensitivity for trichomoniasis is only 60-70%, missing 30-50% of cases 1, 4

Clue Cells

• Epithelial cells covered with bacteria are diagnostic for bacterial vaginosis 2, 1
• Clue cells are usually easily identified in the saline specimen 5
• These bacterial-covered epithelial cells are one of the four Amsel criteria for diagnosing bacterial vaginosis 2, 5

White Blood Cells (WBCs)

• WBCs are quantified as none, few, moderate, or many 1
• Elevated numbers suggest infection and should prompt further investigation 4
• The presence of WBCs on saline microscopy supports a diagnosis of pelvic inflammatory disease when combined with clinical findings 4

KOH Preparation Evaluation

Fungal Elements

• The KOH preparation is examined for yeast cells and pseudohyphae/hyphae indicating Candida species infection 2, 1
• KOH disrupts cellular material that may obscure yeast or pseudohyphae, improving visualization 2
• For vulvovaginal candidiasis diagnosis, wet prep demonstrates yeast or pseudohyphae with normal pH (<4.5) 1

Whiff Test

• The characteristic fishy amine odor produced when KOH is added to secretions is positive in bacterial vaginosis 2, 1
• This is one of the four Amsel criteria for diagnosing bacterial vaginosis 5

Diagnostic Criteria for Specific Infections

Bacterial Vaginosis (Amsel Criteria)

• Three of four findings are required: homogeneous white discharge, pH >4.5, positive whiff test, and clue cells on microscopy 1, 5
• The presence of clue cells on wet mount combined with two other Amsel criteria provides adequate diagnostic accuracy 5

Vulvovaginal Candidiasis

• Diagnosis requires signs and symptoms of vaginitis plus demonstration of yeasts or pseudohyphae on wet preparation 2
• Normal vaginal pH (≤4.5) is associated with Candida infection 2, 1

Trichomoniasis

• Motile trichomonads identified on saline wet mount confirm the diagnosis 1
• Immediate viewing is required for detecting motile trichomonads 1

Critical Timing Considerations

• Wet mount examination should be performed within 30 minutes of collection for optimal results, with a maximum of 2 hours acceptable 1
• Samples stored as wet preparations (swabs in saline) maintain trichomonad motility better than wet mounts (samples on slides), with 20% decrement in motility on slides within 60 minutes 3
• Clinical sites that cannot perform microscopy within 1 hour of sample collection should consider alternative diagnostic tests 3

Common Pitfalls to Avoid

• Do not use Papanicolaou (Pap) test to diagnose T. vaginalis due to poor sensitivity and specificity 1
• The presence of Candida on culture without symptoms should not lead to treatment, as 10-20% of women harbor yeast normally 2, 1
• Gram stain of vaginal secretions, while considered the gold standard for bacterial vaginosis, is not required for clinical diagnosis when Amsel criteria can be applied 5
• Culture of Gardnerella vaginalis is not recommended as it lacks specificity—G. vaginalis can be isolated from half of normal women 5
• Standard wet mount microscopy has limited sensitivity for trichomoniasis; if clinical suspicion is high despite negative wet prep, more sensitive methods like culture or nucleic acid amplification tests should be used 1, 4