Amsel Criteria Components for Bacterial Vaginosis Diagnosis
The Amsel criteria includes four components: homogeneous white vaginal discharge that smoothly coats the vaginal walls, vaginal pH greater than 4.5, positive whiff test (fishy odor with 10% KOH), and presence of clue cells on microscopic examination—with three of four required for diagnosis. 1, 2, 3
The Four Components in Detail
1. Vaginal Discharge Characteristics
- A homogeneous, white, noninflammatory discharge that adheres to (or smoothly coats) the vaginal walls is the first criterion 1, 2
- This discharge is thin and milky in appearance, distinct from the thick white discharge of candidiasis 4, 5
2. Vaginal pH Elevation
- Vaginal fluid pH greater than 4.5 is required 1, 2, 3
- Test pH before applying KOH or performing wet mount, as subsequent testing may alter results 3
- This elevated pH contrasts with the normal vaginal pH of 4.0-4.5 seen in vulvovaginal candidiasis 5
3. Positive Whiff Test
- A fishy odor of vaginal discharge detected before or after addition of 10% KOH (the whiff test) 1, 2, 6
- This amine odor is produced by anaerobic bacteria and has high specificity (99%) for bacterial vaginosis 7
- The whiff test showed 81% sensitivity in pregnant women 7
4. Clue Cells on Microscopy
- The presence of clue cells on microscopic examination of vaginal secretions 1, 2, 3
- Clue cells are vaginal epithelial cells with bacteria adherent to their surface, best visualized in saline wet mount 6
- This criterion has the highest diagnostic performance among individual Amsel components, with 92% sensitivity and 97% specificity 7
Diagnostic Algorithm
Three of the four criteria must be present to diagnose bacterial vaginosis using the Amsel method 1, 2, 4
Sensitivity and Specificity
- The complete Amsel criteria (3 of 4 findings) demonstrates 66.67% sensitivity and 94.74% specificity compared to Gram stain 3
- Individual component performance varies, with clue cells being most reliable and pH being least specific (45% specificity) 7
Common Pitfalls to Avoid
- Do not culture for Gardnerella vaginalis, as it is not specific for BV diagnosis and can be isolated from half of normal women 1, 3
- Ensure microscopic proficiency before relying on wet mount examination, as sensitivity for detecting clue cells varies widely (40-70%) 3
- The presence of vaginal discharge alone is insufficient—approximately 50% of women meeting clinical criteria for BV are asymptomatic 2