Bacterial Vaginosis
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Direct answer
Bacterial vaginosis is a polymicrobial overgrowth of anaerobic bacteria that replaces the normal Lactobacillus flora. It is diagnosed clinically by Amsel criteria, where three of four features are needed, or by a Nugent score of 7 to 10 on Gram stain. It causes a thin, grey, homogeneous discharge with a fishy odour and a vaginal pH above 4.5, and metronidazole is the standard treatment.
What you must remember
- Amsel criteria: Three of four are needed: thin homogeneous discharge, vaginal pH above 4.5, a positive whiff test, and clue cells on at least 20 percent of epithelial cells.
- Nugent score: A Gram stain scored from 0 to 10; 7 to 10 is bacterial vaginosis, 4 to 6 is intermediate and 0 to 3 is normal.
- Clue cells: Vaginal epithelial cells whose borders are obscured by coccobacilli, seen on a saline wet mount.
- Whiff test: An amine odour released when 10 percent potassium hydroxide is added to the discharge.
- Pathogenesis: Loss of Lactobacillus dominance with overgrowth of anaerobes such as Gardnerella vaginalis, Prevotella and Mobiluncus.
- First-line treatment: Oral metronidazole 500 mg twice daily for seven days, or intravaginal metronidazole gel for five days.
- Pregnancy: Symptomatic bacterial vaginosis is treated with metronidazole, while asymptomatic infection is not routinely treated.
- Avoid alcohol: Abstain during oral metronidazole and for at least 24 hours after, because of a disulfiram-like reaction.
A comparison of the two diagnostic systems
Both systems diagnose bacterial vaginosis from the same clinical picture, but they differ in how much laboratory work they need. Examiners often ask which one is the reference standard.
| Feature | Amsel criteria | Nugent score |
|---|---|---|
| Basis | Clinical and bedside tests | Gram stain of a vaginal smear |
| Threshold | Three of four criteria | Score 7 to 10 |
| Lab needed | No, bedside wet mount | Yes, microscopy and scoring |
| Clue cells | Part of the criteria | Not scored directly |
| Typical use | Clinic practice | Research and reference labs |
Amsel criteria suit routine clinic practice because they need only a speculum, pH paper and a wet mount. The Nugent score is the reference standard in research and standardised laboratory work.
A positive whiff test on its own does not establish the diagnosis, since it is only one of four criteria. Either approach is valid when applied consistently and read in the context of the woman's symptoms, and the bedside criteria are usually enough for everyday management.
Where the exam tries to catch you
Bacterial vaginosis questions often test whether you can separate it from candidiasis and handle pregnancy correctly.
- Trap: Diagnosing bacterial vaginosis from pH alone. Correct: pH is one of four Amsel criteria, and the diagnosis needs three of four or a Nugent score of 7 to 10.
- Trap: Calling a white curdy discharge bacterial vaginosis. Correct: Candidiasis gives curdy discharge with normal pH and no fishy odour, whereas bacterial vaginosis gives a thin grey discharge.
- Trap: Routinely screening and treating asymptomatic pregnant women. Correct: Asymptomatic bacterial vaginosis is not routinely treated in pregnancy, while symptomatic women receive metronidazole.
- Trap: Insisting that the sexual partner must be treated. Correct: Routine partner treatment is not recommended for bacterial vaginosis.
Frequently asked questions
How many Amsel criteria are needed to diagnose bacterial vaginosis?
Three of the four Amsel criteria are needed to diagnose bacterial vaginosis. The four are a thin homogeneous discharge, vaginal pH above 4.5, a positive whiff test, and clue cells on at least 20 percent of epithelial cells. Meeting all four is not required.
Which Nugent score range confirms bacterial vaginosis on Gram stain?
A Nugent score of 7 to 10 confirms bacterial vaginosis on Gram stain. A score of 4 to 6 is intermediate, and 0 to 3 indicates normal flora. The score combines Lactobacillus, Gardnerella and Bacteroides morphotypes, and curved Gram-variable rods.
What is the first-line treatment for bacterial vaginosis in a non-pregnant woman?
Oral metronidazole 500 mg twice daily for seven days is a standard first-line regimen for bacterial vaginosis in non-pregnant women. Intravaginal metronidazole gel for five days is an accepted alternative. Clindamycin is an option when metronidazole cannot be used, and alcohol should be avoided during treatment.
Is metronidazole safe for bacterial vaginosis in pregnancy?
Yes, metronidazole is the preferred treatment for symptomatic bacterial vaginosis in pregnancy, in any trimester, per standard guidance. Asymptomatic bacterial vaginosis is not routinely screened for or treated in pregnancy. Oral metronidazole 500 mg twice daily for seven days is the usual regimen.
Why does bacterial vaginosis produce a fishy vaginal odour?
The odour comes from amines such as putrescine and cadaverine, which anaerobic bacteria produce from amino acids. These amines volatilise more readily at an alkaline pH, so semen and menstrual blood intensify the smell. Adding potassium hydroxide to the discharge brings out the same odour in the whiff test.