Spectrum Bias of a Rapid Antigen Detection Test for Group A β-Hemolytic Streptococcal Pharyngitis in a Pediatric Population

BACKGROUND: Rapid antigen detection testing (RADT) is often performed for diagnosis of group A beta-hemolytic streptococcal (GABHS) pharyngitis among children. Among adults, the sensitivity of this test varies on the basis of disease severity (spectrum bias). A similar phenomenon may occur when this test is used in a pediatric population, which may affect the need for culture confirmation of all negative RADT results. OBJECTIVES: To assess the performance of a clinical scoring system and to determine whether RADT spectrum bias is present among children who are evaluated for GABHS pharyngitis. METHODS: Laboratory and clinical records for a consecutive series of pediatric patients who underwent RADT at the Marshfield Clinic between January 2002 and March 2002 were reviewed retrospectively. Patients were stratified according to the number of clinical features present by using modified Centor criteria, ie, history of fever, absence of cough, presence of pharyngeal exudates, and cervical lymphadenopathy. The sensitivity of the RADT was defined as the number of patients with positive RADT results divided by the number of patients with either positive RADT results or negative RADT results but positive throat culture results. RESULTS: RADT results were positive for 117 of 561 children (21%), and culture results were positive for 35 of 444 children (8%) with negative RADT results. The overall prevalence of GABHS pharyngitis was 27% (95% confidence interval: 23-31%). The prevalence of GABHS pharyngitis was 18% among patients with 0 Centor criteria, 16% among those with 1 criterion, 32% among those with 2 criteria, and 50% among those with 3 or 4 criteria. Spectrum bias was present, inasmuch as RADT sensitivity increased with Centor scores, ie, 47% sensitivity among children with 0 Centor criteria, 65% among those with 1 criterion, 82% among those with 2 criteria, and 90% among those with 3 or 4 criteria. CONCLUSIONS: The sensitivity of RADT for GABHS pharyngitis is not a fixed value but varies with the severity of disease. However, even among pediatric patients with > or =3 Centor criteria for GABHS pharyngitis, the sensitivity of RADT is still too low to support the use of RADT without culture confirmation of negative results.