Objective.The sensitivity of a rapid antigen-detection test(RADT)for group A streptococcal(GAS)pharyngitis is critical to whether the test is cost-effe ctive and to whether a confirmatory throat culture is needed.We evaluated a second-generation RADT to determine if its sensitivity varies across the broa d 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 ch art review,we calculated McIsaac scores to separately estimate the pretest clin ical likelihood of GAS pharyngitis for visits at which the RADT result was posit ive(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 sensit ivity was estimated by dividing the number of positive results by the sum of pos itives and false-negatives.Results.As the clinical likelihood of GAS increase d,there were stepwise increases in RADT sensitivity(from 0.67 to 0.88).Sensit ivity was low(0.73;95%confidence interval [CI]:0.62-0.86)in patients clini cally unlikely to have GAS(McIsaac score ≤2)and high(0.94;95%CI:0.89-0.9 9)in patients < 15 years old who had tonsillar exudate and no cough.False-neg ative RADT results were associated with lighter growth of GAS than found on spec imens obtained from a random sample of clinic patients who had only primary thro at cultures ordered.Conclusions.For pediatric patients who are clinically unli kely to have GAS pharyngitis,as indicated by a McIsaac score ≤2,the sensitivi ty of a second-generation RADT may drop below thresholds reported for cost-eff ectiveness.For children who have tonsillar exudate and no cough,the test may b e sensitive enough to meet current pediatric practice guidelines for stand-alon e testing.
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