How Bacterial Vaginosis Is Diagnosed

Confirming Infections vs. Other Causes

The greatest challenge to diagnosing 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 an infection is suspected, a diagnosis can be made with tests that check for bacterial overgrowth in the vagina. 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.

bacterial vaginosis diagnosis
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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 them 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 clear sign of infection, it can be caused by any number other conditions, including trichomoniasis.
  • The second test looks for an enzyme, known as sialidase, which is commonly seen with BV and other forms of vaginitis.

While a negative test is a pretty good indication that you don't have BV, it should not be considered definitive.

In the end, if you have three or more symptoms of BV, you should still see a doctor, especially if they 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 then be analyzed under a microscope to either check for "clue cells" or to confirm the presence of certain bacteria with the use of a gram stain.

Bacterial Vaginosis Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Woman

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 doctor 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, by contrast, is a common technique used to differentiate between groups of bacteria. With BV, certain "good" bacteria would be expected to be low (specifically lactobacilli), while certain "bad" bacteria will be in abundance (usually Gardnerella or Mobiluncus strains). By differentiating these bacteria with dyes and evaluating their proportion under a microscope, doctors can determine whether they meet the criteria for a BV infection.

Clinical Criteria

A doctor 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. Under this criteria, BV can be confirmed when 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 greater 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 type and proportion of bacteria are used to confirm the diagnosis. The diagnosis is based on the following grades:

  • Grade 1: Normal composition of vaginal bacteria
  • Grade 2: Lactobacilli mixed with Gardnerella and/or Mobiluncus bacteria
  • Grade 3: Few lactobacilli and mostly Gardnerella and/or Mobiluncus

Grade 3 can be considered a definitive diagnosis for bacterial vaginosis.

Differential Diagnoses

Because the symptoms of bacterial vaginosis are similar to other infections, doctors may investigate other causes if the test results are borderline or the clinical symptoms are vague.

Differential diagnoses for BV may include:

Frequently Asked Questions

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

While over-the-counter, at-home tests do exist, the most definitive way to diagnose a bacterial vaginosis (BV) infection is to see a doctor. 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 at least three symptoms of BV, be sure to get tested by your nurse or doctor or make an appointment at a sexual health clinic.

How do you test for bacterial vaginosis?

Diagnosing BV involves four steps:

  • A review of your medical history
  • A pelvic exam
  • Performing a pH test to check the level of acidity in the vagina.
  • Taking a sample of vaginal secretions to check for the presence of bacteria by either looking for "clue cells" or using a gram stain technique.

How long will test results for bacterial vaginosis take?

Test results may take anywhere from one to three days, depending on whether your doctor's office is able to examine samples on site or if they must be sent off to a lab.

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.

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Article Sources
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  1. Centers for Disease Control and Prevention. Bacterial Vaginosis (BV) Statistics. Updated February 10, 2020.

  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. 2017;7(3).

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

Additional Reading