Diagnostic Tests for Bacterial Vaginosis
The primary diagnostic tests for bacterial vaginosis are the Amsel clinical criteria and Gram stain (Nugent score), with Gram stain being the gold standard when laboratory facilities are available. 1, 2
Amsel Clinical Criteria (Point-of-Care Testing)
Diagnosis requires meeting 3 of the following 4 criteria:
- Homogeneous white vaginal discharge that smoothly coats the vaginal walls 1, 2
- Vaginal pH >4.5 measured using pH paper strips (test before applying any solutions, as subsequent testing may alter results) 1, 2
- Positive whiff test - characteristic fishy odor when potassium hydroxide (KOH) is added to vaginal secretions due to amines present in the discharge 1, 2
- Clue cells present on saline wet mount microscopy - these are vaginal epithelial cells covered with bacteria 1, 2
The Amsel criteria demonstrate 66.67% sensitivity and 94.74% specificity compared to Gram stain, though reliability in community practice settings varies with examiner experience. 1, 2
Gram Stain (Nugent Score) - Gold Standard
This laboratory test evaluates the relative concentration of bacterial morphotypes in vaginal fluid: 1, 2
- Normal flora shows predominance of large gram-positive rods (Lactobacillus) 2
- BV shows mixed flora with decreased or absent Lactobacillus morphotypes and increased Gardnerella and anaerobes 1, 2
- Specimen collection: Place vaginal swab directly into transport tube, stable at room temperature for 12 hours 1, 2
- Sensitivity ranges from 89% with specificity of 83-95% 2, 3
Molecular Testing (Emerging Standard)
FDA-cleared multiplex nucleic acid amplification tests (NAATs) are now available and offer greater specificity than methods identifying only Gardnerella vaginalis, with 90.5% sensitivity and 85.8% specificity. 2 These tests use lab-provided transport at room temperature for up to 7 days. 1, 2
Specimen Collection Technique
Collect vaginal discharge from the vaginal walls (not the cervix) using a swab: 1, 2
- For wet mount: Submit in 0.5 mL saline or transport swab, room temperature, 2 hours maximum 1
- For Gram stain: Place directly into transport swab tube, room temperature, 12 hours maximum 1, 2
- For DNA testing: Use lab-provided transport, room temperature, up to 7 days 1, 2
Critical Testing Pitfalls to Avoid
- Never culture for Gardnerella vaginalis - it is not specific for BV diagnosis as G. vaginalis can be present in normal vaginal flora 2, 4
- Test vaginal pH before applying KOH or performing wet mount, as subsequent testing may alter results 2
- Wet mount microscopy misses trichomoniasis 30-50% of the time and has variable sensitivity (40-70%) for detecting clue cells, requiring examiner proficiency 1, 2
- The Nugent score requires considerable time and an expert microscopist, making it less practical for immediate clinical decision-making 5