Insensitivity of Rapid Antigen Detection Methods and Single Blood Agar Plate Culture for Diagnosing Streptococcal Pharyngitis

Abstract

To compare the sensitivity of five group A streptococcal antigen detection systems and single blood agar plate culture with a two-plate culture method for diagnosis of streptococcal pharyngitis. Two simultaneous throat swabs were obtained from consecutive patients with suspected streptococcal pharyngitis. One swab was tested for streptococcal antigen by physicians' office nurses and the other was cultured on both aerobic blood agar and anaerobic trimethoprim-sulfamethoxazole blood agar plates. Community office practice and community hospital laboratory. Consecutive outpatients seen by one of four pediatricians or a family practice physician. Results of rapid streptococcal antigen tests were compared with culture results either on a single aerobic blood agar plate or on the two-plate culture method. On throat swabs from 755 consecutive outpatients, the two-plate culture method detected 261 cases (defined as 100%) of group A streptococcal pharyngitis. The anaerobic trimethoprim-sulfamethoxazole plate alone, read at 1 and 2 days, detected 245 cases (94%). The blood agar plate used alone detected 189 cases (72%) at 2 days and 151 cases (58%) at 1 day. Antigen detection test results were positive for 106 throat specimens (41%), with individual kit sensitivity ranging from 31% to 50% compared with the two-plate culture method. Antigen detection test sensitivity decreased with decreasing colony counts. Antigen kit false-positivity rates varied from 0 to 28%. We conclude that the single blood agar plate culture and the antigen detection tests are insensitive, possibly leading the physician toward undertreatment and risking immunologic, local, or distant sequelae. The two-plate culture method should be the standard of practice to rule out streptococcal pharyngitis.