Spectrum and Inoculum Size Effect of a Rapid Antigen Detection Test for Group A Streptococcus in Children with Pharyngitis

Jérémie F. Cohen,Alain Wollner,Edouard Bingen,Franck Thollot,Corinne Levy,Robert Cohen,Martin Chalumeau,Philippe Bidet,Bernard Beall

doi:10.1371/journal.pone.0039085

Abstract

BackgroundThe 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.MethodsThroat 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.ResultsGAS 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.ConclusionsVariations 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.

Highlights

Group A streptococcus (GAS) is found in 20% to 40% of cases of childhood pharyngitis; the remaining cases are considered viral [1]
rapid antigen detection test (RADT) sensitivity is affected by inoculum size [16,17,18] – the amount of group A streptococcus (GAS) colonies identified on throat culture and considered a proxy of the bacterial load on the swab
We aimed to determine whether the diagnostic accuracy of a RADT for GAS pharyngitis is affected by the clinical spectrum effect and/or bacterial inoculum size and to validate a McIsaacscore–based decision rule

Summary

Introduction

Group A streptococcus (GAS) is found in 20% to 40% of cases of childhood pharyngitis; the remaining cases are considered viral [1]. RADTs have high specificity (

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