How Bacterial Vaginosis Is Diagnosed

How Infection Is Confirmed and Differentiated From Other Causes

Lactobacillus crispatus bacteria, illustration
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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 vagina acidity.

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

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 and 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.

While an at-home test can provide the confidentiality and control you seek, they can be quite expensive at roughly $100 per kit.

Labs and Tests

The diagnosis of bacterial vaginosis would typically involve four parts:

  • Your medical history would be reviewed to check whether you have had past vaginal infections, including sexually transmitted ones.
  • A pelvic exam would be performed to visually check for signs of infection.
  • A pH test, performed with a paper test strip, would be 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.

Clue Cells vs. Gram Staining

Clue cells is a term used to describe vaginal cells that, when observed under a microscope, have the characteristics of a bacterial infection. In this instance, the doctor would be looking specifically at epithelial cells (the type that lines hollow organs). If there is a bacterial infection, the edges of these cells would 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 would 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 Diagnosis

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:

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

Sources:

Centers for Disease Control and Prevention (CDC). "Bacterial Vaginosis (BV) Statistics: Bacterial vaginosis is the most common vaginal infection in women ages 15-44." Atlanta, Georgia; updated December 17, 2015.

Hainer, B. and Gibson, M. "Vaginitis: Diagnosis and Treatment." Am Fam Physician. 2011; 83(7):807-815.

Huppert, J.. Hesse, E.; Bernard, M. et al. "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.

Mohammedzadeh, F.; Dolatian, M.; and Jorjani, M. "Diagnostic Value of Amsel’s Clinical Criteria for Diagnosis of Bacterial Vaginosis." Glob J Health Sci. 2015; 7(3):8-14. DOI: 10.5539/gjhs.v7n3p8.