Diagnostic Guidelines for Vaginal Discharge
Point-of-Care Diagnostic Algorithm
The diagnosis of vaginal discharge requires a systematic approach using pH testing, microscopy, and the whiff test to differentiate between the three most common causes: bacterial vaginosis, vulvovaginal candidiasis, and trichomoniasis. 1, 2
Step 1: Measure Vaginal pH
- Apply narrow-range pH paper directly to vaginal secretions (not to the speculum or gloves) 1, 2
- pH >4.5 indicates bacterial vaginosis or trichomoniasis 1, 2
- pH ≤4.5 suggests vulvovaginal candidiasis or physiologic discharge 2
Step 2: Perform the Whiff Test
- Add 10% KOH to vaginal discharge on a slide 1
- A fishy odor immediately after KOH application confirms bacterial vaginosis or trichomoniasis 1, 2
- No odor suggests candidiasis 3
Step 3: Microscopic Examination
Prepare two slides simultaneously:
Saline Wet Mount
- Examine for motile trichomonads (trichomoniasis) 1, 2
- Look for clue cells—vaginal epithelial cells with adherent bacteria obscuring borders (bacterial vaginosis) 1, 3
- Critical pitfall: Wet mount sensitivity for trichomonads is only 40-70%, so negative microscopy does NOT rule out trichomoniasis 1, 3
10% KOH Preparation
- Examine for yeast or pseudohyphae (candidiasis) 1, 2
- KOH dissolves epithelial cells, making fungal elements easier to visualize 1
Step 4: Apply Clinical Diagnostic Criteria
Bacterial Vaginosis (Amsel Criteria)
Diagnosis requires 3 of 4 criteria: 1
- Homogeneous, thin, white-gray discharge adhering to vaginal walls 1, 3
- Clue cells on microscopy 1, 3
- pH >4.5 1, 3
- Positive whiff test (fishy odor with KOH) 1, 3
Alternative: Nugent scoring of Gram stain is more specific than clinical criteria but requires laboratory expertise 1
Vulvovaginal Candidiasis
- Thick, white "cottage cheese-like" discharge 3
- Normal pH (<4.5) 3, 2
- Yeast or pseudohyphae on KOH preparation 1, 3
- Pruritus and vulvar erythema (clinical signs) 4
Trichomoniasis
- Copious, yellow-green, frothy discharge 3
- pH >4.5 3
- Positive whiff test 3
- Motile trichomonads on saline wet mount (when present) 1, 3
Essential Laboratory Testing Beyond Point-of-Care
When Microscopy is Negative or Equivocal
Order NAAT (nucleic acid amplification testing) for Trichomonas vaginalis—do not rely on wet mount alone, as sensitivity is inadequate 1, 3
NAAT for Neisseria gonorrhoeae and Chlamydia trachomatis should be performed, as cervical infections can present with vaginal discharge 3, 5
Culture for Candida species is more sensitive than microscopy but should be reserved for recurrent or complicated cases 1, 4
Special Population Considerations
In postmenopausal women with bloody or tan discharge, endometrial biopsy is essential to rule out endometrial cancer 3
In elderly women, consider atrophic vaginitis (pH typically >4.5 due to loss of lactobacilli) 3
Critical Pitfalls to Avoid
Do not culture for Gardnerella vaginalis—it is present in 50% of normal women and is not specific for bacterial vaginosis 1
Do not treat based on symptoms alone—the characteristics of discharge are non-specific and multiple infections may coexist 1, 5
Do not assume negative wet mount excludes trichomoniasis—NAAT testing is essential if clinical suspicion exists 1, 3
Proficiency in microscopic examination is essential, as infections may be mixed or present with atypical manifestations 1
Laboratory testing fails to identify a cause in a substantial minority of women—consider non-infectious causes like atrophic vaginitis, desquamative inflammatory vaginitis, or vulval dermatoses 1, 5
Diagnostic Workflow Summary
- Collect vaginal discharge specimen with swab 1
- Measure pH with narrow-range paper 1, 2
- Perform whiff test with 10% KOH 1, 2
- Prepare saline wet mount and KOH preparation simultaneously 1
- Examine both slides under microscope at low and high-dry power 1
- If wet mount negative for trichomonads but clinical suspicion high, order NAAT 1, 3
- Consider NAAT for gonorrhea/chlamydia in sexually active women 3
- If all testing negative with persistent symptoms, consider non-infectious causes 5