Antigen detection test for streptococcal pharyngitis: evaluation of sensitivity with respect to true infections

The clinical significance of false-negative results on antigen detection tests for group A beta-hemolytic streptococcal (GABHS) pharyngitis (negative test results and positive culture) has yet to be determined. We recently compared the Culturette Brand Ten-Minute Group A Strep ID Kit with blood agar cultures in 313 patients with pharyngitis, 257 (82%) of whom had positive throat cultures for GABHS. The Culturette Brand test had a sensitivity of 88%, specificity of 96%, a positive predictive value of 99%, and negative predictive value of 64%. More than half of the false-negative Culturette Brand test results occurred in children with less than 10 GABHS colonies on throat culture (1+ culture). When these 1+ cultures were not considered positive, the sensitivity of the Culturette Brand test was 93%. The sensitivity of the Culturette Brand test increased with an increased degree of positivity of the corresponding throat culture. Of the 31 children with false-negative Culturette Brand test results, 14 (45%) had a significant streptococcal antibody response; of the 224 children with true-positive Culturette Brand test results (positive test results and positive culture) from whom serologic data were available, 114 (51%) had a significant streptococcal antibody response. This difference is not statistically significant. These findings suggest that almost half of patients with false-negative results on antigen detection tests for GABHS pharyngitis have true infections (positive culture and antibody rise) and are not merely streptococcal carriers.