This can easily be measured by dipping pH paper into the discharge collected on the speculum. It is not clear whether the increase in pH precedes, and possibly triggers off, the abnormal vaginal flora or occurs as a result of it. This forms the basis of the amine or 'whiff' test, where a drop of KOH is added to a sample of the vaginal fluid on a slide. However positive tests are also found following sexual intercourse and in infection with Trichomonas vaginalis. This is the best method of diagnosis, and is based on the presence of clue cells in the vaginal fluid or secretions.
Clue cells are epithelial cells which are covered by masses of bacteria of varying morphology.
They may be recognised on a Gram stain or by dark-field microscopy of a wet film of vaginal fluid. The curved motile rods of mobiluncus species may also be seen on a wet film. Lactobacilli are decreased, and pus cells are absent. Examination of a Gram-stained vaginal fluid is objective and reproducible with high sensitivity and positive predictive values. With expertise, complete agreement between diagnosis by Gram-stain and composite clinical criteria has been reported.