Detection of group a streptococci by aerobic culture and a new simplified immunoassay in three pediatric practices and a hospital laboratory

Duplicate throat swabs for detection of group A streptococci were collected in three pediatric offices from 1,035 patients with symptoms of pharyngitis. In the collecting office and in the hospital laboratory, one swab from each patient was first inoculated to sheep blood agar (incubated at 35°C aerobically for 2 days) and then tested for group A streptococcal antigen by using the SMART enzyme immunoassay technique (New Horizons Diagnostics Corp) incubated for up to 24 hours. Group A streptococci were recovered in culture (from one or both swabs) and serologically identified from 444 (42.9%) of the patients. Pediatric offices numbers 1, 2, and 3 detected 84.4%, 84.6%, and 82.2%, respectively, of their patients who had positive cultures (in the office and/or laboratory) by using their own culture system and 82.6%, 71.1%, and 84.9%, respectively, of these same patients by using the SMART technique. In the laboratory, SMART test sensitivity and specificity were 71.4% and 98.7%, respectively, after 5 minutes of test incubation. However, SMART test sensitivity improved to 86.5% after overnight incubation of the immunoassay and to 91.3% if the data from one defective lot of seven SMART production lots studied were excluded. SMART test results which are negative after 5 minutes of incubation should therefore be confirmed both by reincubation of the antigen test up to 24 hours and by culture.