Diagnosis of Bacterial Vaginosis
Use the Amsel clinical criteria to diagnose bacterial vaginosis—you need at least 3 of the 4 findings: homogeneous white discharge coating the vaginal walls, vaginal pH >4.5, positive whiff test (fishy odor with KOH), and clue cells on saline wet mount microscopy. 1, 2
Clinical Diagnostic Approach
The CDC explicitly endorses clinical diagnosis without waiting for laboratory confirmation, allowing you to diagnose and treat based on bedside findings alone 1. This is the standard approach in clinical practice.
The Four Amsel Criteria (Need 3 of 4):
- Homogeneous white discharge that smoothly coats and adheres to the vaginal walls 3, 2
- Vaginal pH greater than 4.5 measured with narrow-range pH paper 3, 2
- Positive whiff test: fishy amine odor detected immediately before or after adding 10% KOH solution to vaginal discharge 3, 2
- Clue cells present on microscopic examination of saline wet mount—these are vaginal epithelial cells covered with adherent bacteria (mostly coccobacilli) 3, 4
Alternative Diagnostic Method
Gram stain can be used instead of Amsel criteria by determining the relative concentration of bacterial morphotypes characteristic of BV's altered flora 3, 5. This method is more objective and reproducible with 62-100% sensitivity and 76-100% positive predictive value 6. However, Gram stain is typically reserved for research settings or when microscopy expertise is limited 6.
Critical Diagnostic Pitfalls to Avoid
- Do not culture for Gardnerella vaginalis—this is not recommended because G. vaginalis can be isolated from 50% of normal women, making it completely non-specific 3, 1, 2
- Remember that 50% of women meeting diagnostic criteria are asymptomatic, so the absence of symptoms does not rule out BV 3, 2, 4
- Laboratory testing fails to identify a cause in a substantial minority of women with vaginal symptoms, so negative findings don't always exclude pathology 3, 5
Practical Examination Technique
Perform the wet mount by diluting vaginal discharge in 1-2 drops of 0.9% normal saline on one slide and 10% KOH on a second slide 3. Apply the whiff test by smelling for amine odor immediately after adding KOH 3. Examine both slides under microscope at low- and high-dry power—clue cells are usually easily identified in the saline specimen 3.
When Molecular Testing May Be Considered
While Amsel criteria remain the standard, molecular techniques are gaining relevance when wet mount microscopy is unavailable, when the examiner is inexperienced at recognizing clue cells, or when differential diagnosis of polymicrobial vaginal infections is needed 7, 6. However, these are not required for routine diagnosis.