Improved Diagnostic Performance of an Immunofluorescence-based Rapid Antigen Detection Test for Group A Streptococci in Children With Pharyngitis.

Background: Accurate diagnosis and appropriate treatment of group A streptococcal (GAS) pharyngitis are important to prevent complications. Most available rapid antigen detection tests (RADTs) have shown excellent specificity but often lack sensitivity. Our objective was to compare the diagnostic performances of a new fluorescence-based immunoassay and a classic immunochromatographic RADT using standard throat culture or polymerase chain reaction as references.

Methods: Prospective observational study in 2 pediatric emergency departments in children 3-15 years of age presenting with pharyngitis and a McIsaac score ≥2. Three throat swabs were obtained simultaneously: one for culture and one for each of both RADTs. Polymerase chain reaction assay of the DNaseB sequence was performed in case of discordant results (culture negative and either RADTs positive).

Results: A total of 1002 patients were analyzed, with an overall 37.1% prevalence of GAS pharyngitis. Sensitivity, specificity, positive and negative predictive values were, respectively, 84.9%*, 96.8%, 94.0% and 91.6% for the new fluorescence-based immunoassay, and 75.3%*, 98.1%, 95.9% and 87.0% for the immunochromatographic test (*P < 0.05).

Conclusions: The immunofluorescence-based assay demonstrated improved diagnostic performances over the standard immunochromatographic RADT. Similarly specific for GAS detection, it demonstrates significantly higher sensitivity in children with McIsaac scores 2 or more. A negative result rules out a risk of GAS pharyngitis in 91.6% of children, making it an appropriate tool in pediatric emergency settings. Combined to the low incidence of rheumatic strains, critical appraisal of current practice to routinely perform a backup throat culture from children with pharyngitis and with negative GAS RADT could be reconsidered.