A 29-year-old woman presents to the sexually transmitted infection clinic with concerns regarding a copious...

A 10% potassium hydroxide (KOH) preparation is commonly used in the evaluation of vaginal discharge. In bacterial vaginosis (BV) , adding KOH to a vaginal sample produces a characteristic “whiff test” —a strong fishy amine odor caused by volatilization of amines from anaerobic bacteria (e.g., Gardnerella vaginalis and other anaerobes). MCCQE objectives emphasize recognition of BV as the most common cause of abnormal vaginal discharge in reproductive-age women. Clinical features include thin, gray-white, malodorous discharge without significant inflammation.

KOH microscopy may also reveal clue cells (vaginal epithelial cells coated with bacteria).

Trichomonas vaginalis is diagnosed by saline wet mount showing motile trichomonads or by nucleic acid amplification testing (NAAT), not primarily by KOH. Chlamydia trachomatis and gonorrhea are diagnosed by NAAT. Group B streptococcus is identified by culture.

Thus, use of 10% KOH to confirm a fishy odor (positive whiff test) supports the diagnosis of bacterial vaginosis .

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