External quality control of direct antigen tests to detect group A streptococcal antigen.

Presented here are the results of an external quality control survey organized by the Swiss Center for Quality Control (CSCQ) to evaluate the performance of direct antigen tests (DATs) widely used in Swiss medical practices and laboratories for the diagnosis of group A streptococcal pharyngitis. Twice yearly over a 4-year period, just over 100 participants were requested to analyze positive, weakly positive and negative samples provided to them by the CSCQ with their routinely used DATs and to send the results to the CSCQ. For 1,620 samples distributed, the CSCQ received 1,484 (91.6%) results obtained with 17 different DATs. The specificity of all DATs for negative samples was >91%. For samples containing abundant group A streptococcal antigen, sensitivities ranged from 59.1% to 95.5%; however, for samples containing low levels of antigen, the sensitivity was much lower for all DATs, ranging from 8.7% to 69.8%. Therefore, negative DAT results should be verified with well-performed cultures in order to assure the optimal care of patients with pharyngitis.