Effect of delay in processing on the performance of Directigen for the detection of group A streptococci in throat swabs.

We compared a latex agglutination method, Directigen (Hynson, Westcott & Dunning) with a culture-based system for detection of group A streptococci in 964 throat swabs. Discordants were resolved by Lancefield typing of beta-hemolytic colonies. After culture, swabs were randomly assigned to be tested by Directigen within 4 h of sampling or after overnight storage. For swabs that yielded more than 50 beta-hemolytic colonies from culture, there was a highly significant difference (P less than 0.01) in sensitivity between the group tested early and the group tested after storage overnight. For swabs tested within 4 h of sampling, a trend (P = 0.07) existed between sensitivity and length of delay before testing.