According to Amsel's criteria for diagnosing bacterial vaginosis (BV), how many criteria must be met for a diagnosis?

Amsel's criteria for diagnosing bacterial vaginosis requires that at least three out of four specific criteria are met to establish a diagnosis. These criteria include the presence of a homogeneous, thin, grayish-white vaginal discharge, a vaginal pH greater than 4.5, the whiff test (a fishy odor when 10% KOH is added to vaginal secretions), and the presence of clue cells on microscopic examination of vaginal fluid. Meeting at least three of these criteria helps to distinguish bacterial vaginosis from other types of vaginitis, such as candidiasis or trichomoniasis, which have different clinical features and treatment approaches. When the majority of these criteria are satisfied, it provides a higher level of confidence in the diagnosis of BV.