Clinical evaluation of a latex agglutination test for streptococcal pharyngitis: performance and impact on treatment rates

The accuracy and clinical utility of a latex agglutination test were compared with anaerobic throat culture on selective media for detection of Group A streptococcal pharyngitis in an urban pediatric emergency department. Among 255 symptomatic cases prevalence of positive culture was 29% and antigen test sensitivity was 55%. Among 100 asymptomatic controls prevalence of positive culture was 20% and antigen test sensitivity was 20%. Controls with positive cultures and cases with false negative antigen tests had significantly fewer colonies on culture than cases with true positive antigen tests (P less than 0.01). Symptoms and clinical findings were not associated with antigen test result or number of colonies on culture. Eighty percent of patients with positive cultures received treatment when the antigen test was used as an adjunct to culture, compared with a 57% treatment rate among the subgroup in whom follow-up treatment was attempted based on positive culture results alone (P less than 0.05). We conclude that: (1) the antigen test had lower sensitivity in routine clinical use than previously reported; (2) the high rate of false negative tests may result, in part, from a high proportion of specimens with low colony counts; and (3) the availability of the antigen test as an adjunct to culture significantly increased treatment rates.