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Objective: To determine the validity of the rapid antigen test for the diagnoses of acute pharyngitis caused by group A beta-haemolytic Streptococcus (GABHS) compared with culture., Design: Observational study of a consecutive sample of paediatric patients., Setting: Two primary care centres (PCC) from the metropolitan area of Barcelona., Participants: Children aged 1-14 years with sore throat of no more than 5 days duration were chosen at PCC., Principal Measurements: Oropharyngeal samples were collected from tonsillar bed and posterior pharynx. A rapid diagnostic test was performed, as well as a throat culture., Results: A total of 211 patients were studied. The overall prevalence of pharyngitis due to Streptococcus was 34.1%. Compared with the throat culture, the sensitivity of the rapid test was 90.3% (95% CI: 81.0-96.0), the specificity was 78.4% (95% CI: 70.6-84.9). The percentage of false negatives was 9.7% and the false positives was 21.6%. Spectrum bias was present, inasmuch as the rapid test sensitivity increased with Centor scores., Conclusions: The diagnostic value of a rapid antigen test for the diagnosis of streptococcal pharyngitis in paediatric patients at PCC is high. However, the percentage of false positives and negatives is too high, and also the sensitivity is too low in patients with fewer symptoms to support the use of rapid antigenic test without culture confirmation and bacterial sensitivity test., (2009 Elsevier España, S.L. All rights reserved.)