Bacterial vaginosis (BV) is a common vaginal condition resulting from an imbalance in the naturally occurring bacteria within the vagina. This involves a decrease in beneficial bacteria and an increase in other types. Diagnosing this bacterial imbalance frequently involves examining vaginal fluid under a microscope. Microscopic analysis directly observes the changes in the vaginal environment that characterize BV.
Preparing the Sample for Microscopic View
Preparing a vaginal fluid sample is the first step in microscopic BV diagnosis. A healthcare professional typically collects the sample using a swab from the vaginal walls during an examination. The collected fluid contains cells and bacteria for analysis.
Once collected, the sample can be prepared in a few ways for viewing. A common method is the wet mount, where a small amount of vaginal discharge is mixed with a drop of normal saline on a microscope slide. This allows immediate viewing of cellular structures and bacteria in their natural state. A separate sample portion may also receive a drop of 10% potassium hydroxide (KOH). This helps dissolve epithelial cells, reveal fungal elements, and is used for the “whiff test” to detect a characteristic odor.
The Gram stain is another preparation method, involving spreading vaginal fluid thinly on a slide, air drying, and then applying stains. This process differentiates bacteria by cell wall composition, categorizing them as Gram-positive (purple) or Gram-negative (pink/red) and allowing observation of their distinct shapes. It is useful for assessing the overall bacterial population and identifying specific morphotypes.
What the Microscope Reveals
Microscopic examination of the prepared vaginal fluid provides visual clues for bacterial vaginosis. A key finding is “clue cells,” vaginal epithelial cells densely covered with bacteria, making their borders appear obscured or “fuzzy.” These cells are a specific marker for BV, as adherent bacteria, primarily Gardnerella vaginalis and other anaerobes, are visible.
The bacterial flora shows a distinct pattern in BV. In a healthy vaginal environment, Lactobacillus species (large Gram-positive rods) are abundant. In BV, however, these beneficial lactobacilli are noticeably reduced or absent. There is an overgrowth of various anaerobic bacteria, including small Gram-variable coccobacilli (Gardnerella vaginalis) and curved rods (Mobiluncus species).
Microscopic observation also notes white blood cells (leukocytes). In BV, white blood cells are typically absent or in very low numbers. This paucity of inflammatory cells helps distinguish BV from other vaginal infections that trigger a more pronounced immune response, such as yeast infections or trichomoniasis.
Making a Diagnosis from Microscopic Findings
Microscopic observations are central to diagnosing BV, often relying on established criteria. The Amsel Criteria are four clinical signs used to diagnose BV; at least three must be present for a confirmed diagnosis. One criterion directly involves microscopic findings: clue cells on a saline wet mount. Other Amsel criteria, such as a vaginal pH greater than 4.5 and a positive “whiff test” (fishy odor after adding potassium hydroxide), provide supporting evidence for BV.
The Nugent Score offers a more detailed, microscopy-based diagnostic system, useful with Gram-stained slides. This system quantifies bacterial morphotypes in the sample. Points are assigned based on the abundance of large Gram-positive rods (lactobacilli), small Gram-variable rods (Gardnerella and Bacteroides), and curved Gram-variable rods (Mobiluncus).
A score is calculated based on the proportions of these bacterial types. Scores of 0-3 indicate normal flora, 4-6 suggest intermediate flora, and 7-10 are diagnostic of BV. The Nugent score provides a standardized, objective method to assess bacterial shifts characteristic of BV, offering a comprehensive microscopic evaluation.