How Bacterial Vaginosis Is Diagnosed

Confirming Infections vs. Other Causes

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The greatest challenge to getting a diagnosis of bacterial vaginosis (BV) is that the majority of cases will have no symptoms, according to a report from the Centers for Disease Control and Prevention. If you or your healthcare provider think that you may have an infection, tests that check for bacterial overgrowth in the vagina can confirm your diagnosis. The evaluation would also include a pelvic exam, an analysis of vaginal secretions, and a pH test to check for vaginal acidity.

In addition to lab tests, there are self-testing kits that allow you to check your vaginal pH and other markers of inflammation at home.

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At-Home Tests

Bacterial vaginosis is characterized by symptoms of vaginal discharge, itchiness, burning, and a characteristic "fishy" odor. The symptoms can be easily mistaken for a yeast infection and are often inappropriately treated as such.

To differentiate, some women will test themselves by using an at-home test purchased online or from a local drug store.

The test, which has been available over the counter since 2001, is relatively accurate and may provide enough evidence to seek a definitive diagnosis and treatment from a clinic.

With that being said, the existing tests don't actually test for BV. Instead, they look for changes in vaginal acidity and byproducts of a BV infection.

The test is performed in two parts:

  • The first test looks for evidence of an elevated vaginal pH. While a high pH is considered a sign of infection, it can be caused by any number other conditions, including trichomoniasis (a parasite that can cause a vaginal infection).
  • The second test looks for an enzyme known as sialidase, which is commonly seen with BV and other forms of vaginitis.

A negative test is an indication that you don't have BV, but it should not be considered definitive.

In the end, if you have three or more symptoms of BV, you should see a healthcare provider even if your home test is negative, especially if your symptoms are severe, persistent, or recurrent.

Labs and Tests

The diagnosis of bacterial vaginosis typically involves four parts:

  • Your medical history is reviewed to check whether you have had past vaginal infections, including sexually transmitted ones.
  • A pelvic exam is performed to visually check for signs of infection.
  • A pH test, performed with a paper test strip, is used to measure vaginal acidity. A pH of over 4.5 is a strong indication of a bacterial infection.
  • Vaginal secretions would be sampled and examined with a microscope to check for inflammatory cells and to assess the presence of certain bacteria with the use of a gram stain.

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Clue Cells vs. Gram Staining

Clue cells describe vaginal cells that, when observed under a microscope, have the characteristics of a bacterial infection. In this instance, the healthcare provider is looking specifically at epithelial cells (the type that line hollow organs). If there is a bacterial infection, the edges of these cells will be peppered with bacteria. Their fuzzy appearance would provide the "clues" needed to help make the diagnosis.

Gram staining is a common technique used to differentiate between groups of bacteria. With BV, bacteria that are part of the normal vaginal flora would be expected to be low (specifically lactobacilli), while harmful bacteria will be in abundance (usually Gardnerella or Mobiluncus strains). By differentiating these bacteria with dyes and evaluating their proportion microscopically, healthcare providers can determine whether you meet the criteria for a BV infection.

Clinical Criteria

A healthcare provider can make a definitive diagnosis of bacterial vaginosis with one of two evaluative measures: the Amsel criteria or gram stain grading.

The Amsel criteria take into account the physical findings alongside the result of the diagnostic tests.

Using these criteria, BV can be confirmed when three of the following four conditions are met:

  • There is a whitish or yellow vaginal discharge.
  • Clue cells are seen under the microscope.
  • The vaginal pH is higher than 4.5.
  • There is a release of a fishy odor when an alkali solution is added to vaginal secretions.

Gram staining is an alternate method in which the number of different types of bacteria is used to determine a Nugent score that ranges from 0-10.

The diagnosis is based on the following system:

  • The number of Lactobacilli, Gardnerella, and Mobiluncus bacteria in a microscopic field are counted. Each type corresponds to a scale with a score between one to 4 (Mobiluncus bacteria is scored from one to three).
  • The score for each type of bacteria is added for a total ranging from zero to 10.

Grades seven through 10 can be considered a definitive diagnosis for bacterial vaginosis.

The CDC recommends diagnosing BV either based on Amsel's criteria or by determining the Nugent score from a gram stain.

Differential Diagnoses

Because the symptoms of bacterial vaginosis are similar to other infections, healthcare providers may investigate other causes if the test results are borderline or the clinical symptoms raise concerns about another condition.

Differential diagnoses for BV may include:

  • Candidiasis (a yeast infection caused by the Candida fungus)
  • Trichomoniasis (a sexually transmitted infection caused by Trichomonas vaginalis)
  • Herpes simplex virus (HSV) of the genitals
  • Other sexually transmitted infections, such as chlamydia or gonorrhea
  • Aerobic vaginitis (a condition similar to BV but that is caused by bacteria foreign to the vagina, including Staphylococcus aureus and Escherichia coli)
  • Cervicitis (inflammation of the cervix with both infectious and non-infectious causes)

Frequently Asked Questions

  • Can I use an at-home test to diagnose bacterial vaginosis?

    You can use an at-home test to look for bacterial vaginosis (BV), but the most definitive way to diagnose a BV infection is by seeing a healthcare provider. At-home tests look for byproducts of BV infection, but not the presence of the bacteria themselves, which means that you could get a false-negative result. If you have symptoms of BV, be sure to get tested by your nurse or healthcare provider or make an appointment at a sexual health clinic.

  • How do you test for bacterial vaginosis?

    You can test for bacterial vaginosis by using a home testing kit or by visiting a healthcare provider. They may do the following:

    • Review your medical history.
    • Perform a pelvic exam.
    • Use a pH test to check the level of acidity in the vagina.
    • Take a sample of vaginal secretions to check for the presence of bacteria by either looking for "clue cells" or using a gram stain technique.
  • Can I test for bacterial vaginosis when I'm on my period?

    It's best to wait until after your period to test for BV, as blood can make it harder to get the clear sample necessary to check your vaginal fluids for bacteria.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Bacterial Vaginosis (BV) Statistics.

  2. Huppert JS, Hesse EA, Bernard MC, Bates JR, Gaydos CA, Kahn JA. Accuracy and trust of self-testing for bacterial vaginosis. J Adolesc Health. 2012;51(4):400-5. doi:10.1016/j.jadohealth.2012.01.017

  3. Huppert JS, Hesse EA, Bernard MC, Bates JR, Gaydos CA, Kahn JA. Accuracy and trust of self-testing for bacterial vaginosis. J Adolesc Health. 2012;51(4):400-5. doi:10.1016/j.jadohealth.2012.01.017

  4. Centers for Disease Control and Prevention. Bacterial Vaginosis

  5. Antonucci FP, Mirandola W, Fontana C. Comparison between Nugent's and Hay/Ison scoring criteria for the diagnosis of Bacterial Vaginosis in WASP prepared vaginal samples. Clinical Investigation.

  6. U.S. National Library of Medicine. Medline Plus. Vaginitis.

Additional Reading

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.