title: Relationship between the clinical likelihood of group a streptococcal pharyngitis and the sensitivity of a rapid antigen-detection test in a pediatric practice.
abstract: 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 < or =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 < or =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.
authors.affiliation: Division of General Pediatrics and Adolescent Medicine, Department of Pediatrics, University of Wisconsin Medical School, Madison, Wisconsin, USA. mbedmons@wisc.edu
authors.forename: M Bruce
authors.initials: MB
authors.lastname: Edmonson
authors.forename: Kathryn R
authors.initials: KR
authors.lastname: Farwell
mesh_terms: D000293:Adolescent; D017677:Age Distribution; D000942:Antigens, Bacterial / Q000032:analysis*; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007118:Immunoassay; D007223:Infant; D008297:Male; D015999:Multivariate Analysis; D010612:Pharyngitis / Q000382:microbiology*; D010614:Pharynx / Q000382:microbiology; D015995:Prevalence; D011933:Reagent Kits, Diagnostic; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D017678:Sex Distribution; D016037:Single-Blind Method; D013290:Streptococcal Infections / Q000175:diagnosis* / Q000382:microbiology; D013297:Streptococcus pyogenes / Q000276:immunology / Q000302:isolation & purification*
chemical_list: D000942:Antigens, Bacterial; D011933:Reagent Kits, Diagnostic