Health Encyclopedia



Tests for Bacterial Vaginosis

Test Overview

Tests for bacterial vaginosis take samples of fluid and cells from the vagina to see if signs of infection are present.

Bacterial vaginosis is caused by a change in the balance of microorganisms found in a healthy vagina. A healthy vagina normally has many microorganisms in it. The microorganisms involved in bacterial vaginosis include Gardnerella, Mobiluncus, Bacteroides, and Mycoplasma. When bacterial vaginosis is present, these microorganisms increase in number while the number of healthy microorganisms decrease.

Many women with bacterial vaginosis do not have symptoms. The most common symptom of bacterial vaginosis is an increase in vaginal discharge. The discharge often has a fishy smell.

Pregnancy risk

Women who have bacterial vaginosis during pregnancy have a higher chance of miscarriage, early (preterm) delivery, and an infection after delivery, so it is important for pregnant women who have symptoms to be tested for bacterial vaginosis.


Several tests can be used to find bacterial vaginosis:

  • Wet mount. A sample of vaginal discharge is mixed with a salt solution on a microscope slide. The slide is checked for bacteria, white blood cells, and unusual cells called clue cells. If clue cells are present, it means bacterial vaginosis may be present.
  • Whiff test. Several drops of a potassium hydroxide (KOH) solution are added to a sample of vaginal discharge to see whether a strong fishy odor is produced. A fishy odor on the whiff test usually means bacterial vaginosis is present.
  • Vaginal pH. The normal vaginal pH is 3.8 to 4.5. Bacterial vaginosis often causes the vaginal pH to be greater than 4.5.
  • Oligonucleotide probes. This test finds the genetic material (DNA) of this bacteria. An oligonucleotide probe test is very accurate but is not routinely used to diagnose bacterial vaginosis..

Bacterial vaginosis may be found during a Pap test. But a Pap test is not recommended as a test to find bacterial vaginosis.

Why It Is Done

Tests for bacterial vaginosis are done to help find the cause of an abnormal vaginal discharge or other symptoms of a vaginal infection, such as vaginal irritation or pain.

How To Prepare

Do not douche, have sex, or use vaginal medicines for 24 hours before having a bacterial vaginosis test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

You will take off your clothes below the waist. You will have a gown to drape around your waist. You will then lie on your back on an examination table with your feet raised and supported by stirrups. This is similar to having a pelvic examination or Pap test.

Your doctor will insert a lubricated tool called a speculum into your vagina. The speculum gently spreads apart the vaginal walls, allowing your doctor to see the inside of the vagina and the cervix.

Samples of fluid inside the vagina are then collected with a swab or wooden stick.

How It Feels

You may feel some discomfort when the speculum is put in, especially if your vagina is irritated and tender.


There is little or no risk in having a bacterial vaginosis test.


Tests for bacterial vaginosis take samples of fluid and cells from the vagina to see if signs of infection are present.

A high vaginal pH, clue cells, and a fishy odor are some signs that bacterial vaginosis is present.

Tests for bacterial vaginosis

No abnormal vaginal discharge is present on vaginal exam.

A wet mount does not show large numbers of bacteria, such as Gardnerella, that cause bacterial vaginosis.

Few or no clue cells are present.

No fishy odor is present when a potassium hydroxide (KOH) solution is added to a sample of vaginal discharge.

Vaginal pH is in the normal range of 3.8 to 4.5.


A bacterial vaginosis infection is present.

  • A thin, grayish white vaginal discharge is present on vaginal exam. The discharge often looks shiny and has small bubbles.
  • A fishy odor is made when a KOH solution is added to a sample of vaginal discharge.
  • Large numbers of the types of bacteria that cause bacterial vaginosis (such as Gardnerella), clue cells, or both are present on wet mount.
  • Vaginal pH is greater than 4.5.


What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • If you are having your menstrual period.
  • If you use a vaginal medicine.
  • Having sex or douching 24 hours before this test.

What To Think About

  • Bacterial vaginosis during pregnancy increases a woman's risk of early (preterm) delivery. Because of this risk, it is important to treat bacterial vaginosis during pregnancy with antibiotics.
  • Experts disagree about calling bacterial vaginosis a sexually transmitted infection (STI). A history of STIs or having multiple sex partners increases the chance of developing bacterial vaginosis.


Other Works Consulted

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.


By Healthwise Staff
Primary Medical Reviewer Sarah Marshall, MD - Family Medicine
Specialist Medical Reviewer Devika Singh, MD, MPH - Infectious Disease
Last Revised August 18, 2013

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