woman scientist looking through a microscope

Which Lab Test Can Detect Bacterial Vaginosis?

Bacterial vaginosis is a prevalent vaginal infection that is identified using a range of procedures, such as the Whiff test, pH test, microscopic inspection, Nucleic Acid Amplification Tests (NAATs), and Amsel Criteria. These tests detect the presence of microorganisms, evaluate vaginal pH levels, and analyze vaginal discharge for certain attributes. A positive diagnosis usually requires the presence of a fishy odor, an increased pH level, and the existence of clue cells. Prompt identification and intervention are crucial for preserving vaginal well-being and averting problems.

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Bacterial vaginosis is a common excess of the bacteria in the vaginal area. To treat and keep your vaginal health, you need to find out what kind of problem you have. To find out if someone has bacterial vaginosis, doctors use the Whiff test, the pH test, a microscope, Nucleic Acid Amplification Tests (NAATs), and the Amsel Criteria.

woman scientist looking through a microscope

All of the tests are needed to get a full picture of the illness and help doctors make smart choices about how to care for patients.

Vaginal Health Panel

Tests for bacterial vaginosis usually include analyzing vaginal discharge for signs of infection. Common techniques for identifying bacterial vaginosis include:

An easy way to tell if someone has bacterial vaginosis is to do a whiff or amine test. When vaginal fluid is mixed with potassium hydroxide, it gives off a fishy smell, which is how the term “Whiff test” came about.

  • Sample Collection: A healthcare worker uses a swab to get a small amount of vaginal waste, usually from the back of the vagina.
  • Preparation: A drop of a 10% potassium hydroxide (KOH) solution is added to the sample after the swab is placed on a glass slide. Because KOH is alkaline, it helps get rid of the amines, which are chemical substances that bacteria make and are linked to bacterial vaginosis.
  • Observation: The medical expert continues to sniff the mixture. Someone will smell something strong and fishy right after the KOH is added if a good result is found. When bacteria that cause bacterial vaginosis break down, they create volatile amines like putrescine and cadaverine, which give the urine its smell.

The pH test checks how acidic or basic the vaginal discharge is, which is very important for vaginal health.1 The pH test finds bacterial vaginosis by showing that the vaginal environment is more alkaline.

This is how the pH test generally works:

  • Sample Collection: Vaginal discharge is collected by a clinician during sample collection. A gently sterile swab inserted and removed from the vagina is common.
  • pH Measurement: Sample pH is tested with a strip or paper. The pH-sensitive dye in these strips changes color with sample acidity or alkalinity. Color charts indicate pH and color variations.
  • Interpretation: Vaginal discharge is normally 3.8–5 acidic.2 Vaginal infections like bacterial vaginosis may occur at pH exceeding 5. In bacterial vaginosis, bacteria overgrow, diminishing vaginal acidity-keeping lactobacilli. Alkaline pH.

Samples need to be looked at under a microscope to confirm bacterial vaginosis. A sample of vaginal fluid is looked at under a microscope to find germs and changes in cells that are linked to BV. Full breakdown of the process:

  • Sample Collection: The doctor takes a clean swab of the discharge from the uterus. Since most of the discharge is focusing on the vaginal walls, a sample is taken.
  • Slide Preparation: The object is put on a glass slide in a flat pattern and told to dry naturally. A saline solution could be added to the sample to help the lens see the details of the cells better.
  • Staining: To color dried samples, you use Gram stain. This method helps figure out what germs and cells are in a sample. Gram stain testing helps find clue cells that show that a woman has bacterial vaginosis.
  • Microscopic Examination: The dyed slide is looked at in the sun by a doctor or lab tech. The following traits will be looked for:
    • Clue Cells: These vaginal epithelial cells are cloudy or grainy because bacteria cover them. Clue cells are very important for diagnosing bacterial vaginosis.3
    • Lactobacilli: Lactobacilli that are long and rod-shaped are what make up a healthy vaginal environment.4 Less lactobacilli may be a sign of an imbalance in the vaginal flora, which is common in BV.
    • Other Bacteria: Gardnerella vaginalis, a rod-shaped bacteria, can also be a sign of BV.
  • Interpretation: The results of the microscopic test are judged by looking at the patient’s symptoms and other medical factors. A positive Whiff test, clue cells, and an increase in vaginal pH can all show that you have bacterial vaginosis.

These molecular diagnostic tests find DNA or RNA from bacteria and viruses. By recognizing their DNA or RNA, NAATs can find bacterial vaginosis bugs.

An in-depth look at NAATs for bacterial vaginosis:5

  • Sample Collection: A doctor swabs fluid from the uterus. Put the sample in a shipping medium to keep the genetic material safe until it can be tested.
  • Extraction: To get the bacteria’s genetic material out of the sample, a chemical method is used. This method separates the genetic material from the sample, which makes it easier to find.
  • Amplification: NAATs need to take genetic material out and make it bigger. A piece of bacterial DNA or RNA is copied with enzymes. A popular way to increase the amount of NAAT is to use polymerase chain reaction.
  • Identification: To find increased genetic material, different types of NAAT use different methods. This could be done with probes that show when they link to certain genetic material or visible dyes that stick to DNA or RNA.
  • Interpretation: Bacteria that cause bacterial vaginosis are shown by positive test results. If the readings are not positive, it means that these germs are not present.

The Amsel Diagnostic Criteria tells doctors how to tell if someone has bacterial vaginosis. It was made by the team led by Dr. Robert P. Amsel in the 1980s. If you meet three of these four conditions, you may have bacterial vaginosis:

  • Homogeneous Vaginal Discharge: Vaginal discharge that is homogeneous is thin and white, and it covers the walls evenly. The vaginal discharge that changes with each period is different from the “milky” or “creamy” discharge.
  • Vaginal pH Greater than 5: Most of the time, vaginal pH is between 3.8 and 5. If it’s above 5, it’s bad. A pH level above 5 means the air is alkaline, which is good for bacterial vaginosis bacteria.
  • Positive Whiff Test: When the vaginal fluid is mixed with 10% potassium hydroxide (KOH), the test is positive. A fishy smell from volatile amines made by bacteria means things will go well.
  • Presence of Clue Cells on Microscopic Examination: Vaginal epithelial cells that are covered in bacteria are hint cells. Bacteria make the surface of cells rough or stippled, which makes it hard to see the edges of cells. Bacterial vaginosis is shown by finding clue cells.


To treat bacterial vaginosis and stop it from getting worse, it is important to get a correct diagnosis. The Whiff test, the pH test, microscopic examination, NAATs, and the Amsel Criteria are all used to identify this common vaginal illness. Healthcare professionals need to know how to do these medical tests so they can give patients the right treatment at the right time, which will improve their health and well-being.

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Lab Testing - Frequently Asked Questions

Why is it important to do lab tests occasionally?

It is important to do lab tests occasionally because they can provide valuable information about an individual's health and help to identify potential health issues early on. Lab tests can measure a wide range of factors, including blood count, cholesterol levels, liver and kidney function, and hormone levels, and can provide insight into an individual's overall health and wellness. Additionally, lab tests can help to diagnose and monitor the progression of certain medical conditions, such as diabetes and heart disease, and can help to identify any potential health risks or concerns. By doing lab tests occasionally, individuals can take proactive steps to maintain their health and wellbeing and reduce the risk of potential health problems in the future.

What does a routine blood test cover?

A routine blood test is used to check for a range of things, including your blood count and the levels of certain chemicals and substances in your blood. Blood tests can also be used to check how well certain organs, such as your liver and kidneys, are functioning.

How is a blood sample collected for lab testing?

A blood sample for lab testing is typically collected through a process called venipuncture, which involves inserting a small needle into a vein to draw blood. This is usually done on the inside of the elbow or the back of the hand.

What is the cost of a lab test?

In general, the cost of a lab test can range from a few dollars to several hundred dollars. It is always best to consult with your doctor or healthcare provider to get an accurate estimate of the cost of a lab test.

Read More: Lab Testing FAQ


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[2] Lin YP, Chen WC, Cheng CM, Shen CJ. Vaginal pH Value for Clinical Diagnosis and Treatment of Common Vaginitis. Diagnostics (Basel). 2021 Oct 27;11(11):1996. doi: 10.3390/diagnostics11111996. PMID: 34829343; PMCID: PMC8618584.;

[3] Cook RL, Reid G, Pond DG, Schmitt CA, Sobel JD. Clue cells in bacterial vaginosis: immunofluorescent identification of the adherent gram-negative bacteria as Gardnerella vaginalis. J Infect Dis. 1989 Sep;160(3):490-6. doi: 10.1093/infdis/160.3.490. PMID: 2668431.;

[4] Tannock GW. A special fondness for lactobacilli. Appl Environ Microbiol. 2004 Jun;70(6):3189-94. doi: 10.1128/AEM.70.6.3189-3194.2004. PMID: 15184111; PMCID: PMC427720.;

[5] Danby CS, Althouse AD, Hillier SL, Wiesenfeld HC. Nucleic Acid Amplification Testing Compared With Cultures, Gram Stain, and Microscopy in the Diagnosis of Vaginitis. J Low Genit Tract Dis. 2021 Jan 1;25(1):76-80. doi: 10.1097/LGT.0000000000000576. PMID: 33347046.;