Diagnostic Testing for Bacterial Vaginosis
The gold standard for diagnosing bacterial vaginosis is the Nugent Gram stain scoring system performed on a vaginal swab, though the Amsel clinical criteria (requiring 3 of 4 findings) is the most practical bedside approach and is recommended by the CDC for routine clinical diagnosis. 1
Primary Diagnostic Methods
Amsel Clinical Criteria (Bedside Diagnosis)
Diagnose BV when 3 of the following 4 criteria are present: 2
- Homogeneous white discharge that smoothly coats the vaginal walls 2
- Vaginal pH >4.5 (test pH before applying KOH or performing wet mount, as subsequent testing may alter results) 1
- Positive whiff test (fishy odor before or after addition of 10% KOH) 2
- Clue cells present on microscopic examination of wet mount 2
The Amsel criteria demonstrates 66.67% sensitivity and 94.74% specificity compared to Gram stain. 1 However, ensure microscopic proficiency before relying on wet mount examination, as sensitivity for detecting clue cells varies widely (40-70%). 1
Gram Stain Scoring (Laboratory Gold Standard)
The Nugent criteria evaluates the relative concentration of bacterial morphotypes in vaginal fluid: 1
- Normal flora shows predominance of large gram-positive rods (Lactobacillus) 1
- BV shows mixed flora with decreased or absent Lactobacillus morphotypes 1
- Vaginal discharge swabs for Gram stain should be placed directly into transport tubes, stable at room temperature for 12 hours 1
This is the most specific procedure available and serves as the reference standard against which other tests are compared. 1
Emerging Molecular Testing Options
FDA-cleared microbiome-based multiplex NAATs are now available and offer greater specificity than methods identifying only Gardnerella vaginalis, with 90.5% sensitivity and 85.8% specificity. 1 These molecular tests provide reproducible and standardized results, are objective, can detect fastidious bacteria, enable quantitation, and are ideal for self-collected vaginal swabs. 3, 4
For DNA testing, use lab-provided transport at room temperature for up to 7 days. 1
Point-of-Care Testing
The BVBlue test is a chromogenic diagnostic test based on elevated sialidase enzyme in vaginal fluid, demonstrating 88% sensitivity and 95% specificity compared to Nugent scoring, and 88% sensitivity and 91% specificity compared to Amsel criteria. 5, 6 This is particularly useful in settings without microscopy capabilities. 5
Critical Testing Pitfalls to Avoid
- Never culture for Gardnerella vaginalis as it is not specific for BV diagnosis—G. vaginalis can be present in normal vaginal flora 1
- Test vaginal pH before applying KOH or performing wet mount 1
- Ensure microscopic proficiency exists before relying solely on wet mount examination 1
When to Test
Test all women with symptomatic vaginal discharge, odor, or irritation. 1 Consider testing asymptomatic high-risk pregnant women and before invasive gynecological procedures (surgical abortion, hysterosalpingography, IUD placement) to prevent serious ascending infections including post-procedure pelvic inflammatory disease, vaginal cuff cellulitis, and endometritis. 1, 7
Remember that up to 50% of women meeting clinical criteria for BV are completely asymptomatic, meaning absence of symptoms does not exclude the diagnosis or eliminate the risk of complications. 7