How to Test for Bacterial Vaginosis
Diagnose BV using the Amsel clinical criteria, which requires three of the following four findings: homogeneous white non-inflammatory discharge coating the vaginal walls, clue cells on microscopy, vaginal pH >4.5, and a positive whiff test (fishy odor with 10% KOH). 1, 2
Clinical Diagnostic Approach (Amsel Criteria)
The CDC recommends clinical diagnosis as the primary method, requiring three of four criteria 1:
Vaginal discharge characteristics: Look for a thin, homogeneous, white or grayish-white discharge that smoothly coats the vaginal walls without inflammatory signs 1
Vaginal pH measurement: Use narrow-range pH paper applied to vaginal secretions (can dip directly into discharge on speculum tip after removal). A pH >4.5 suggests BV or trichomoniasis 1
Whiff test: Mix vaginal discharge with 10% potassium hydroxide (KOH) solution. A fishy amine odor immediately upon mixing indicates BV 1
Microscopy for clue cells: Prepare a wet mount by diluting vaginal discharge in 1-2 drops of normal saline on a glass slide. Clue cells are epithelial cells with bacteria obscuring their borders, and >20% of epithelial cells should be clue cells 1, 3
Alternative Laboratory Method
Gram stain (Nugent scoring) can be used as an alternative diagnostic standard, evaluating the relative concentration of bacterial morphotypes characteristic of BV 1, 2. A Nugent score ≥4 indicates BV, while 0-3 represents normal flora 4. This method assesses markedly reduced or absent Lactobacillus morphology and predominance of Gardnerella morphotype 4.
Point-of-Care Testing Options
Newer commercial tests are available that measure pH and detect amines or sialidase activity, which may be helpful when microscopy is unavailable 1, 5. The BVBlue test demonstrated 88% sensitivity and 95% specificity compared to Nugent scoring 6.
Critical Diagnostic Pitfalls
Do not culture Gardnerella vaginalis for diagnosis—it can be isolated from half of normal women and lacks specificity 1
Read wet mounts immediately after preparation to maximize detection of organisms, particularly if also evaluating for trichomonads 1
Up to 50% of women with BV are asymptomatic, so absence of symptoms does not exclude the diagnosis 1, 2, 7
Symptoms alone are inadequate for diagnosis: vaginal discharge has only 5% specificity and malodor only 12% specificity compared to microscopy 8
When to Test
The CDC recommends testing before invasive gynecologic procedures (surgical abortion, hysterosalpingography, IUD placement) even in asymptomatic women, as BV increases risk of post-procedure infections including endometritis, pelvic inflammatory disease, and vaginal cuff cellulitis 2, 7.