title: Spectrum and inoculum size effect of a rapid antigen detection test for group A streptococcus in children with pharyngitis.
abstract: BACKGROUND
  The stability of the accuracy of a diagnostic test is critical to whether clinicians can rely on its result. We aimed to assess whether the performance of a rapid antigen detection test (RADT) for group A streptococcus (GAS) is affected by the clinical spectrum and/or bacterial inoculum size.
  
  
  METHODS
  Throat swabs were collected from 785 children with pharyngitis in an office-based, prospective, multicenter study (2009-2010). We analysed the effect of clinical spectrum (i.e., the McIsaac score and its components) and inoculum size (light or heavy GAS growth) on the accuracy (sensitivity, specificity, likelihood ratios and predictive values) of a RADT, with laboratory throat culture as the reference test. We also evaluated the accuracy of a McIsaac-score-based decision rule.
  
  
  RESULTS
  GAS prevalence was 36% (95CI: 33%-40%). The inoculum was heavy for 85% of cases (81%-89%). We found a significant spectrum effect on sensitivity, specificity, likelihood ratios and positive predictive value (p<0.05) but not negative predictive value, which was stable at about 92%. RADT sensitivity was greater for children with heavy than light inoculum (95% vs. 40%, p<0.001). After stratification by inoculum size, the spectrum effect on RADT sensitivity was significant only in patients with light inoculum, on univariate and multivariate analysis. The McIsaac-score-based decision rule had 99% (97%-100%) sensitivity and 52% (48%-57%) specificity.
  
  
  CONCLUSIONS
  Variations in RADT sensitivity only occur in patients with light inocula. Because the spectrum effect does not affect the negative predictive value of the test, clinicians who want to rule out GAS can rely on negative RADT results regardless of clinical features if they accept that about 10% of children with negative RADT results will have a positive throat culture. However, such a policy is more acceptable in populations with very low incidence of complications of GAS infection.
authors: Unité de Recherche Epidémiologique en Santé Périnatale et Santé des Femmes et des Enfants (U953), Inserm, Paris, France. jeremie.cohen@inserm.fr
authors: Jérémie F
authors: JF
authors: Cohen
authors: Martin
authors: M
authors: Chalumeau
authors: Corinne
authors: C
authors: Levy
authors: Philippe
authors: P
authors: Bidet
authors: Franck
authors: F
authors: Thollot
authors: Alain
authors: A
authors: Wollner
authors: Edouard
authors: E
authors: Bingen
authors: Robert
authors: R
authors: Cohen
mesh_terms: D000293:Adolescent; D000942:Antigens, Bacterial / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D003657:Decision Making; D019937:Diagnostic Techniques and Procedures; D005260:Female; D005602:France / Q000453:epidemiology; D006801:Humans; D008297:Male; D010612:Pharyngitis / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology*; D015995:Prevalence; D011933:Reagent Kits, Diagnostic; D013297:Streptococcus pyogenes / Q000254:growth & development* / Q000276:immunology* / Q000302:isolation & purification
chemical_list: D000942:Antigens, Bacterial; D011933:Reagent Kits, Diagnostic