How Is BV Tested? Swabs, Home Kits and pH Strips Explained

Bacterial vaginosis (BV) is diagnosed on a vaginal swab or an examination, not a urine test. A GP or sexual health clinic looks at the discharge, checks its acidity and reads or sends a swab, and a laboratory grades the bacteria. A home pH strip shows only that the acidity has risen, which happens in BV and trichomoniasis but not thrush, so a strip can prompt a swab and cannot replace one.^1-3 The testing page of the Wellgard BV guide; is it thrush, BV or something else? compares the conditions.

How is BV tested in the UK, and where do I go?

The NHS says see a GP or a sexual health clinic if you think you have BV, and to seek treatment in pregnancy.¹ The clinician "will want to confirm it is bacterial vaginosis and rule out an STI", may examine you, and "a cotton bud may be wiped over the discharge inside your vagina".¹ Many clinics are walk-in and "often get test results quicker than GP surgeries"; no NHS page gives a turnaround in days.¹ A GP may not test at all: NICE's clinical knowledge summary says that with typical symptoms "examination and further tests may be omitted and empirical treatment started" if the woman is at low STI risk, has no other symptoms and is not pregnant, among other conditions.⁴ Otherwise, a swab.

What does a high vaginal swab test for?

A high vaginal swab is a long cotton bud wiped over the discharge in the upper vagina and examined for the bacterial pattern of BV, for yeast (thrush) and for the parasite behind trichomoniasis; late in pregnancy the NHS says it is "possible to be tested for group B strep".^{1, 5, 6} All three matter: in a study of 1,740 women with symptoms, 20% had two infections at once and 24% had none of the three.⁷ A BV swab is not an STI screen; BV and a positive chlamydia test covers the overlap.^{1, 6}

What are the Amsel criteria and the Hay-Ison grades?

The clinic method comes from a 1983 study of 397 students: three of four signs make the diagnosis. A thin, homogeneous grey-white discharge; a pH above 4.5; a fishy smell when a drop of alkali meets the sample (the "whiff test"); and "clue cells", vaginal skin cells coated in bacteria, under the microscope.² That study grew Gardnerella from 51.3% of all the women, so a report naming it is not by itself a diagnosis.² UK clinics mostly grade a Gram-stained smear instead (the Hay-Ison grades): grade I lactobacilli (normal), grade II mixed (intermediate), grade III the BV pattern. In the UK validation study of 162 clinic visits the grading agreed with the Amsel criteria with a sensitivity of 97.5% and a specificity of 96%; 12 of 13 intermediate smears came from women who did not meet the clinical criteria.³ Research laboratories use the Nugent score on the same stain: 7 or more out of 10 is BV.⁸ An intermediate or "mixed" report is read beside your symptoms.

What does pH have to do with it?

A lactobacillus-dominated vagina makes close to 1% lactic acid and sits at a pH of about 3.8 when measured in place.¹² In BV the lactobacilli fall and the pH rises above 4.5, the one Amsel criterion a strip can copy.⁴ But trichomoniasis raises it too, thrush leaves it alone, and menstrual blood and semen are alkaline, so a strip read around a period or after sex reads high anyway.^{4, 10} Vaginal flora and pH explains why the acid is there.

Do home BV tests and pH strips work?

They measure one thing and diagnose nothing. A strip reads pH; a "BV test kit" adds a bacterial-enzyme test. In the one independent study of 131 women aged 14 to 22, against a clinical diagnosis the strip was 73% sensitive and 67% specific: it caught about three in four BV cases and wrongly flagged a third of the women without BV; the enzyme test was 40% sensitive and 90% specific.⁹ No strip can separate BV from trichomoniasis, see clue cells or rule out an STI. In pooled studies of 18,424 women, those with BV were about twice as likely to go on to acquire trichomoniasis (adjusted odds ratio 1.87), an STI that also raises pH and in which, the NHS says, "many people get no symptoms at all".^{6, 13} A raised reading is a reason for a clinic swab, a normal one does not rule BV out (the strip missed one case in four), and the NHS says "do not self-diagnose".^{9, 14}

How do I take a vaginal swab myself?

Self-taken swabs are standard. The NHS trichomoniasis page says "you may be able to do the swab yourself", and in the 1,740-woman study self-collected swabs performed like clinician-collected ones.^{6, 7} The clinic's leaflet says how far to insert it and how long to turn it; the swab goes back in its tube to the same laboratory, so a clinic self-swab is not a home kit. Because blood and semen raise pH, you may be asked to return after a period or recent sex.⁴

Does BV show in a urine test, and is it a UTI?

No, and no. A urinary tract infection is a bacterial infection of the bladder or urethra, felt as "pain or a burning sensation when peeing" and needing to pee more often, diagnosed on a urine sample ("although this is not always needed"), with an antibiotic where needed.¹¹ BV is a shift in the vaginal bacteria, felt as a thin grey discharge and a fishy smell, diagnosed on a swab and "usually treated with antibiotic tablets or gels or creams".¹ Thrush is a yeast, felt as itch and soreness with a white discharge "which does not usually smell", diagnosed on a swab; the NHS says "you'll usually need antifungal medicine".⁵ The evidence for cranberry, D-mannose and live cultures belongs to one condition each and does not transfer; a urine dipstick and culture sets it out in one line each, and what BV discharge looks like takes the vaginal side. A negative urine test with discharge has not finished the testing.

What happens after a positive test?

Antibiotics, "prescribed by a GP or sexual health clinic".¹ A positive BV test is followed by an antibiotic; no strip, kit or supplement changes that. How BV is treated in the UK covers the routes. In pregnancy a positive test goes to the GP or midwife: the NHS notes "a small chance of complications" and that BV "causes no problems in the majority of pregnancies".¹ If symptoms persist after the course, NICE says "the diagnosis of BV should be reconsidered": another swab, not another gel.⁴ Why BV keeps coming back covers the NHS threshold of more than four episodes a year.¹

Get help from NHS 111 if your discharge changes colour, smell or texture, is heavier than usual, is itchy or sore, or comes with bleeding between periods or after sex, pain when peeing or pelvic pain.¹⁴ See a GP or sexual health clinic for a first episode, in pregnancy, for fever or pelvic pain, if treatment has not worked, for BV more than four times a year or thrush more than four times in 12 months, after a new partner, for a first episode of thrush under 16 or over 60, and for any bleeding, sores or skin change.^{1, 5, 14} What your discharge means covers the rest.

Frequently asked questions

Can you test for BV at home?

You can test the pH. A strip caught about three in four BV cases and wrongly flagged a third of women without BV, and no strip separates BV from trichomoniasis.⁹ The swab is the test.

How long do swab results take?

No NHS page gives a number of days. The NHS says sexual health clinics "often get test results quicker than GP surgeries".¹ Ask how you will be told.

Can a swab be wrong, or say "mixed growth"?

Rarely: in the UK validation study the smear agreed with the clinical criteria in 96% of BV cases. A "mixed" or intermediate report means the flora sits between the two patterns and is read with your symptoms; if symptoms persist, the test is repeated.^{3, 4}

Is BV tested for in a standard STI screen?

Not necessarily. BV is not an STI, and the NHS describes confirming BV and ruling out an STI as two tasks, so ask for the BV swab.¹ Is BV an STI? covers the classification.

Sources

1. NHS. Bacterial vaginosis. Reviewed 19 June 2026. nhs.uk/conditions/bacterial-vaginosis.
2. Amsel R, et al. Nonspecific vaginitis: diagnostic criteria and microbial and epidemiologic associations. American Journal of Medicine, 1983. PubMed 6600371.
3. Ison CA, Hay PE. Validation of a simplified grading of Gram stained vaginal smears for use in genitourinary medicine clinics. Sexually Transmitted Infections, 2002. PubMed 12473800.
4. NICE Clinical Knowledge Summaries. Bacterial vaginosis. Last revised July 2023. cks.nice.org.uk/topics/bacterial-vaginosis.
5. NHS. Thrush in men and women. Reviewed 28 July 2023. nhs.uk/conditions/thrush-in-men-and-women.
6. NHS. Trichomoniasis. Reviewed 16 July 2025. nhs.uk/conditions/trichomoniasis; NHS. Infections in pregnancy that may affect your baby. Reviewed 9 April 2024. nhs.uk/pregnancy/keeping-well/infections-that-may-affect-your-baby.
7. Gaydos CA, et al. Clinical validation of a test for the diagnosis of vaginitis. Obstetrics and Gynecology, 2017. PubMed 28594779.
8. Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation. Journal of Clinical Microbiology, 1991. PubMed 1706728.
9. Huppert JS, et al. Accuracy and trust of self-testing for bacterial vaginosis. Journal of Adolescent Health, 2012. PubMed 22999842.
10. Shen CJ, et al. Clinical evaluation of a self-testing kit for vaginal infection diagnosis. Journal of Healthcare Engineering, 2021. PubMed 34394891.
11. NHS. Urinary tract infections (UTIs). Reviewed 11 July 2025. nhs.uk/conditions/urinary-tract-infections-utis.
12. O'Hanlon DE, et al. Vaginal pH measured in vivo: lactobacilli determine pH and lactic acid concentration. BMC Microbiology, 2019. PubMed 30642259.
13. Seña AC, et al. Bacterial vaginosis and its association with incident Trichomonas vaginalis infections: a systematic review and meta-analysis. Sexually Transmitted Diseases, 2021. PubMed 34433796.
14. NHS. Vaginal discharge. Reviewed 15 February 2024. nhs.uk/conditions/vaginal-discharge.

This article is general information, not medical advice. A first episode, any change in pregnancy, bleeding, pain or fever means a GP or sexual health clinic, not a strip.