Published online February 1, 2005
PEDIATRICS Vol. 115 No. 2 February 2005, pp. 280-285 (doi:10.1542/peds.2004-0907)
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Relationship Between the Clinical Likelihood of Group A Streptococcal Pharyngitis and the Sensitivity of a Rapid Antigen-Detection Test in a Pediatric Practice

M. Bruce Edmonson, MD, MPH and Kathryn R. Farwell, BS

From the Division of General Pediatrics and Adolescent Medicine, Department of Pediatrics, University of Wisconsin Medical School, Madison, Wisconsin

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 ≤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.

Key Words: pharyngitis • streptococcal infections • rapid diagnostic tests

Abbreviations: RADT, rapid antigen-detection test • GAS, group A streptococcus • TC, throat culture • CI, confidence interval • AOR, adjusted odds ratio

Accepted Jul 19, 2004.




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