Relationship between the clinical likelihood of group a streptococcal pharyngitis and the sensitivity of a rapid antigen-detection test in a pediatric practice

Objective. The sensitivity of a rapid antigen-detection test (RADT) for group A streptococcal ( GAS) pharyngitis is critical to whether the test is cost-effective and to whether a confirmatory throat culture is needed. We evaluated a second-generation RADT to determine if its sensitivity varies across the broad clinical spectrum of patients tested for GAS in pediatric outpatient practice. Methods. We used laboratory logbooks from a single pediatric clinic to identify 1184 consecutive patient visits at which an RADT was performed. In a blinded chart review, we calculated McIsaac scores to separately estimate the pretest clinical likelihood of GAS pharyngitis for visits at which the RADT result was positive ( n = 384) and for visits at which the result proved to be false-negative ( n = 65). Positive RADT results were assumed to be true positives, and test sensitivity was estimated by dividing the number of positive results by the sum of positives and false-negatives. Results. As the clinical likelihood of GAS increased, there were stepwise increases in RADT sensitivity ( from 0.67 to 0.88). Sensitivity was low ( 0.73; 95% confidence interval [CI]: 0.62 - 0.86) in patients clinically unlikely to have GAS ( McIsaac score less than or equal to 2) and high (0.94; 95% CI: 0.89 - 0.99) in patients < 15 years old who had tonsillar exudate and no cough. False-negative RADT results were associated with lighter growth of GAS than found on specimens obtained from a random sample of clinic patients who had only primary throat cultures ordered. Conclusions. For pediatric patients who are clinically unlikely to have GAS pharyngitis, as indicated by a McIsaac score < 2, the sensitivity of a second-generation RADT may drop below thresholds reported for cost-effectiveness. For children who have tonsillar exudate and no cough, the test may be sensitive enough to meet current pediatric practice guidelines for stand-alone testing.