Bacterial Vaginosis (BV) is a common condition resulting from an imbalance in the natural bacterial population of the vagina. A healthy vaginal environment is typically dominated by beneficial Lactobacillus species. With BV, these are displaced by an overgrowth of other bacteria, such as Gardnerella vaginalis. This shift in vaginal flora can lead to symptoms, including a noticeable odor and unusual discharge. Receiving a negative test result despite having classic BV symptoms is a frequent dilemma that highlights the limitations of current diagnostic methods.
How Bacterial Vaginosis is Diagnosed
A medical professional uses clinical observations and laboratory tests to establish a BV diagnosis. The most widely used method is the Amsel Criteria, which requires at least three of four specific signs to be present. These criteria include:
- A thin, homogeneous gray-white discharge.
- A vaginal pH level greater than 4.5.
- The presence of “clue cells” seen on a microscopic wet mount.
- A positive “whiff test,” which involves adding potassium hydroxide (KOH) solution to the discharge, resulting in a fishy odor.
The gold standard for laboratory diagnosis is the Nugent Scoring system, which uses a Gram stain of vaginal fluid to assess the bacterial balance. This method quantifies the relative numbers of different bacteria, looking for the displacement of protective Lactobacillus species by anaerobic bacteria. A score is calculated based on the types and amounts of bacteria present, with a score of seven or higher indicating BV.
Molecular tests, such as Nucleic Acid Amplification Tests (NAATs) or Polymerase Chain Reaction (PCR) tests, are also becoming common. These tests directly detect the genetic material of specific BV-associated bacteria. Molecular diagnostics offer high sensitivity and specificity by identifying multiple organisms linked to the condition, providing a more objective measure than microscopic or clinical methods.
Factors Contributing to Inaccurate Test Results
A negative result in the presence of symptoms, known as a false negative, can occur due to factors related to sample quality and test limitations. Errors in the sample collection process can significantly skew the results. For example, engaging in sexual intercourse, douching, or using vaginal medications within 24 to 48 hours before the test can disrupt the bacterial balance or dilute the sample, leading to an inaccurate reading.
The inherent limitations of diagnostic methods also contribute to inaccuracies. The Amsel criteria rely on subjective interpretation of discharge and odor. Nugent scoring requires a trained microbiologist to accurately count and identify bacterial types, introducing variability that can lead to misclassification. The Nugent system may also categorize a sample as “intermediate” (a score between 4 and 6), which indicates abnormal flora but leaves the diagnosis uncertain.
Molecular testing is highly accurate but has limitations. Some assays only screen for a narrow list of BV-associated microbes. Since BV is polymicrobial, involving an overgrowth of several different bacteria, a test targeting only one species may miss the diagnosis if the infection is driven by other organisms.
The timing of the test relative to the infection cycle or recent medication use can also suppress the bacterial load below the detection threshold. Recent use of antibiotics, antifungals, or certain probiotics can temporarily suppress the overgrowth, causing a negative test even if the underlying dysbiosis persists. Testing too early in the course of an infection, before the bacterial load has significantly increased, may also result in a false negative.
Other Conditions with Similar Symptoms
If a BV test is truly negative, the symptoms are likely caused by a different vaginal condition that shares similar discharge or irritation.
Yeast Infection (Candidiasis)
A common alternative diagnosis is a yeast infection, or candidiasis, caused by an overgrowth of the Candida fungus. Unlike BV, which typically produces a thin, gray or white discharge and a fishy odor, a yeast infection often results in a thick, white, “cottage cheese”-like discharge and usually lacks an odor.
Trichomoniasis
Another possibility is trichomoniasis, a sexually transmitted infection (STI) caused by the parasite Trichomonas vaginalis. Trichomoniasis can mimic BV by causing a frothy, sometimes greenish-yellow discharge and a foul odor. Distinguishing between BV and trichomoniasis is crucial because they require different treatments.
Cytolytic Vaginosis (CV)
A differential diagnosis is Cytolytic Vaginosis (CV), which involves an overgrowth of protective Lactobacillus bacteria. This excessive amount of lactic acid leads to an abnormally low vaginal pH, causing symptoms like discharge, burning, and irritation. Simple allergic reactions or chemical irritations, known as contact dermatitis, from products like soaps or detergents can also cause redness, itching, and burning without an underlying infection.
When to Seek Further Medical Evaluation
When symptoms persist despite a negative BV test, a thorough medical re-evaluation is necessary to determine the correct underlying cause. The first step involves an open discussion with a healthcare provider about the persistent symptoms. In some cases, a clinician may choose to treat the symptoms based on clinical presentation, a practice known as clinical correlation, especially if the patient meets some, but not all, of the Amsel criteria.
It is wise to repeat the test after a short period, ensuring all pre-test instructions, such as avoiding douching or sex, are strictly followed to prevent sampling errors. If symptoms are severe or recurrent, requesting a different type of test, such as a molecular assay, may provide a more definitive answer by looking for a broader range of microbes. Persistent vaginal symptoms also warrant ruling out sexually transmitted infections, as many share overlapping signs with BV.
For individuals experiencing persistent or frequently recurring symptoms, consulting a specialist, such as a gynecologist, is advisable. A specialist can perform a more complex differential diagnosis, which may include testing for less common conditions like Cytolytic Vaginosis or Desquamative Inflammatory Vaginitis (DIV). A persistent problem with a negative BV test requires continued partnership with a medical professional to find an accurate solution.