Wet mount microscopy reflects functional vaginal lactobacillary flora better than Gram stain

The status of vaginal lacto-bacillary flora, an indicator of possible genital infection and pregnancy complications, can be assessed on wet mount or Gram stained specimens. The former is quick, the latter more routine. The accuracy of the two preparative techniques to detect normal vaginal lacto-bacillary microflora was compared for 646 patients. The effect of delay in transport medium before Gram staining was also investigated.

Patients presented with infectious vaginitis or for a routine prenatal visit. After placement of a speculum, duplicate smears were taken from the upper vaginal vault and examined fresh or after Gram staining. Lacto-bacillary grades from both methods were compared with lactate concentration in vaginal rinses. In a subgroup of 238 patients, Gram staining was performed both on fresh smears and those that had been transported in Stuart's growth medium.

Higher lacto-bacillary grades (more disrupted flora) were diagnosed 2.9 times more frequently on Gram stained specimens than on wet mounts (p < 0.0001), a difference even more pronounced after transport in Stuart's medium (relative risk, 4.2; p < 0.0001). Lacto-bacillary grades assessed on wet mounts correlated better with vaginal lactate concentration than those assessed on Gram stains.

Easier recognition of lacto-bacillary morphotypes on wet mounts than on Gram stains might result from the loss of lactobacilli by the process of fixation or Gram staining. Wet mount microscopy of vaginal smears for assessment of lacto-bacillary grades, rather than Gram staining, is strongly recommended.