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Lab Test That Can Detect Bacterial Vaginosis

Bacterial vaginosis is a common vaginal infection that is diagnosed using many methods, including the Whiff testpH test, microscopic examination, Nucleic Acid Amplification Tests (NAATs), and Amsel Criteria.

These tests ascertain the existence of bacteria, assess vaginal pH levels, and scrutinize vaginal discharge for certain characteristics. A definitive diagnosis often necessitates the presence of a pungent odor resembling that of fish, an elevated pH level, and the presence of clue cells. Timely recognition and action are essential for maintaining vaginal health and preventing complications.

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Vaginal Health Panel

Bacterial vaginosis tests often include examining vaginal discharge for indications of infection. Typical methods used to detect bacterial vaginosis include:

A simple method to determine the presence of bacterial vaginosis in an individual is by doing a whiff or amine test. The phrase “Whiff test” originated from the observation that when vaginal fluid is combined with potassium hydroxide, it emits a malodorous scent resembling that of fish.

  • Sample Collection: A healthcare professional utilizes a swab to collect a tiny quantity of vaginal discharge, often from the posterior region of the vagina.
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  • Preparation: After placing the swab on a glass slide, a small amount of a 10% potassium hydroxide (KOH) solution is applied to the sample. KOH, which is alkaline in nature and linked to bacterial vaginosis, aids in the removal of amines.
  • Observation: The doctor keeps inhaling the concoction. If a positive outcome is achieved, the addition of KOH will immediately produce a pungent and distinct fish-like odor. When the bacteria that cause bacterial vaginosis break down, volatile amines like putrescine and cadaverine are released.

The pH test assesses the acidity or alkalinity of the vaginal discharge, a crucial factor in maintaining vaginal health.1 The pH test detects bacterial vaginosis by indicating a higher alkalinity in the vaginal environment.

The pH test typically operates in the following manner:

  • Sample Collection: A doctor obtains vaginal discharge during the process of sample collection. The act of inserting and removing a sterile swab from the vagina is a routine procedure.
  • pH Measurement: A pH sample is analyzed using a strip or paper. The pH-sensitive dye included in these strips undergoes a color shift in response to the acidity or alkalinity of the sample. Color charts display pH levels and changes in color.
  • Interpretation: The pH level of normal vaginal discharge typically ranges from 3.8 to 5, indicating acidity.2 Vaginal infections, such as bacterial vaginosis, can arise when the pH values in the vagina surpass 5. Bacterial vaginosis occurs when there is excessive growth of bacteria, leading to a decrease in the acidity of the vagina, which in turn reduces the presence of lactobacilli.

A microscopic examination is necessary to conclusively diagnose bacterial vaginosis. An examination of vaginal fluid under a microscope is conducted to identify microorganisms and cellular alterations associated with bacterial vaginosis (BV).

  • Sample Collection: The doctor obtains a sterile sample of the uterine discharge. A sample is obtained as the majority of the discharge is concentrated on the vaginal walls.
  • Slide Preparation: The piece is placed onto a glass slide in a horizontal arrangement and let to air-dry. To enhance the lens’s ability to observe cellular features, it is possible to introduce a saline solution into the sample.
  • Staining: The process of coloring dried samples involves the utilization of Gram stain. This approach facilitates the identification of microorganisms and cells present in a sample. Gram stain testing is used to detect clue cells, which indicate the presence of bacterial vaginosis in women.
  • Microscopical examination: A medical professional or laboratory technician inspects the pigmented slide outdoors in sunlight. The following characteristics will be sought after:
  • Clue Cells: The vaginal epithelial cells seem hazy or grainy due to bacterial coverage. Clue cells play a crucial role in the diagnosis of bacterial vaginosis.3
  • Lactobacilli: The healthy vaginal environment is composed of elongated and rod-shaped Lactobacilli.4 A decrease in lactobacilli levels may indicate an imbalance in the vaginal flora, a condition frequently observed in bacterial vaginosis (BV).
  • Other Bacteria: Gardnerella vaginalis, a bacterium with a rod-like structure, can also indicate the presence of bacterial vaginosis (BV).
  • Interpretation: The microscopic test findings are evaluated based on the patient’s symptoms and other relevant medical criteria. A positive Whiff test, the presence of clue cells, and an increase in vaginal pH can all help diagnose bacterial vaginosis.

These molecular diagnostic assays detect the presence of bacterial and viral DNA or RNA. NAATs can detect bacterial vaginosis pathogens by identifying their DNA or RNA.

A comprehensive examination of nucleic acid amplification tests (NAATs) for the detection of bacterial vaginosis:5

  • Sample Collection: A doctor collects a sample of fluid from the uterus using a swab. Place the sample in a suitable shipping medium to ensure the preservation of the genetic material until it can undergo testing.
  • Extraction: A chemical approach is employed to extract the genetic material of the bacterium from the sample. This technique facilitates the identification of genetic material by isolating it from the sample.
  • Amplification: Nucleic Acid Amplification Tests (NAATs) involve extracting genetic material and subsequently amplifying it. An enzymatic process is used to replicate a segment of bacterial DNA or RNA. One often employed method to enhance the quantity of NAAT is by the utilization of polymerase chain reaction.
  • Identification: Different types of nucleic acid amplification tests (NAATs) employ distinct methodologies to detect an augmented amount of genetic material. This can be achieved using probes that exhibit affinity for certain genetic material or visible dyes that adhere to DNA or RNA.
  • Interpretation: Positive test findings indicate the presence of microorganisms responsible for bacterial vaginosis. If the values are negative, it indicates the absence of certain microorganisms.

The Amsel Diagnostic Criteria provides guidelines for physicians to determine the presence of bacterial vaginosis in individuals. Dr. Robert P. Amsel headed a team that created it during the 1980s. If you satisfy three out of these four criteria, it is possible that you have bacterial vaginosis:

  • Homogeneous Vaginal Discharge: The vaginal discharge that is homogenous appears as a thin and white substance, which uniformly coats the walls of the vagina. The vaginal discharge that varies with each menstrual cycle is distinct from the “milky” or “creamy” discharge.
  • Vaginal pH Greater than 5: The majority of the time, the pH level in the vagina falls between the range of 3.8 to 5. If the value exceeds 5, it is considered unfavorable. A pH level above 5 indicates alkalinity in the air, which is conducive to the growth of germs associated with bacterial vaginosis.
  • Positive Whiff Test: A good result is obtained when the vaginal fluid is combined with a 10% solution of potassium hydroxide (KOH). If there is a presence of volatile amines produced by bacteria, it indicates a positive outcome.
  • Presence of Clue Cells on Microscopic Examination: Hint cells refer to vaginal epithelial cells that are coated with bacteria. Bacteria induce a rough or stippled texture on the surface of cells, hence impeding the visibility of cell boundaries. Detecting clue cells is indicative of bacterial vaginosis.


In order to effectively manage bacterial vaginosis and prevent its progression, obtaining an accurate diagnosis is crucial. The Whiff test, pH test, microscopic inspection, nucleic acid amplification tests (NAATs), and the Amsel Criteria are employed for the identification of this prevalent vaginal ailment. Healthcare workers must possess the knowledge and skills to perform these medical tests in order to administer appropriate therapy to patients promptly, therefore enhancing their overall health and well-being.

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[1] Hemalatha R, Ramalaxmi BA, Swetha E, Balakrishna N, Mastromarino P. Evaluation of vaginal pH for detection of bacterial vaginosis. Indian J Med Res. 2013 Sep;138(3):354-9. PMID: 24135180; PMCID: PMC3818598.

[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.

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