How is bacterial vaginosis diagnosed?

How to Test for Bacterial Vaginosis

Bacterial vaginosis should be diagnosed using either the Amsel clinical criteria (requiring 3 of 4 findings) or Gram stain scoring, with Gram stain being the gold standard when available. 1

Primary Diagnostic Approaches

Amsel Clinical Criteria (Point-of-Care Method)

Diagnose BV when 3 of the following 4 criteria are present: 1, 2

• Homogeneous white discharge that smoothly coats the vaginal walls 1, 2
• Vaginal pH >4.5 (measured with narrow-range pH paper) 1, 2
• Positive whiff test (fishy odor when 10% KOH is added to vaginal discharge) 1, 2
• Clue cells present on microscopic examination of saline wet mount 1, 2

The Amsel criteria can be performed while the patient is in the office and provides immediate results, though it has lower specificity than Gram stain. 1 Studies show the Amsel criteria has 66.67% sensitivity and 94.74% specificity compared to Gram stain, with the whiff test having the lowest correlation with true BV. 3

Gram Stain Scoring (Laboratory Gold Standard)

Gram stain is more specific than all other diagnostic methods and should be used when laboratory capabilities exist. 1

• Evaluates the relative concentration of bacterial morphotypes in vaginal fluid 1
• Normal flora shows predominance (3-4+) of large gram-positive rods (Lactobacillus morphotype) 4
• BV shows mixed flora with decreased or absent (0-2+) Lactobacillus morphotypes and increased small gram-variable rods (Gardnerella morphotype) 4
• Requires a vaginal discharge swab placed directly into transport tube, stable at room temperature for 12 hours 1

Newer Molecular Diagnostic Options

DNA-Based Testing

Molecular tests detecting Gardnerella vaginalis DNA or vaginal sialidase activity have similar sensitivity and specificity to Gram stain and are increasingly available. 5, 6

• DNA hybridization probes can be performed on vaginal swabs, stable at room temperature for 7 days 1
• Sialidase enzyme detection (e.g., BVBlue test) showed 91.7% sensitivity and 97.8% specificity compared to Gram stain 7
• Nucleic acid amplification tests (NAATs) are increasingly used in clinical practice 6

Specimen Collection and Transport

Collect vaginal discharge using a swab from the vaginal walls: 1

• For wet mount examination: submit in 0.5 mL saline or transport swab, room temperature, within 2 hours 1
• For Gram stain: place directly into transport swab tube, room temperature, within 12 hours 1
• For DNA testing: use lab-provided transport, room temperature, up to 7 days 1

Critical Testing Pitfalls to Avoid

Do not culture for Gardnerella vaginalis—it is not specific for BV diagnosis. 1 G. vaginalis can be present in normal vaginal flora, making culture unreliable. 1

Ensure microscopic proficiency before relying on wet mount examination. 1 Wet mount sensitivity for detecting clue cells varies widely (40-70%), and infections may be mixed or present atypically. 1

Test vaginal pH before applying KOH or performing wet mount. 1 The pH strip should be applied to vaginal discharge first, as subsequent testing may alter results. 1

When to Test

Test all women with symptomatic vaginal discharge, odor, or irritation. 2 However, recognize that up to 50% of women meeting diagnostic criteria for BV are asymptomatic. 2

Consider testing asymptomatic high-risk pregnant women (those with prior preterm delivery) before invasive gynecological procedures, as BV is associated with adverse pregnancy outcomes and post-procedural infections. 1, 2