we evaluate the test characteristics and test for spectrum bias of a rapid antigen test for group a beta hemolytic streptococcal gabhs pharyngitis among adults medical record and laboratory results of consecutive adult patients receiving a rapid antigen test for gabhs in the emergency department or urgent care clinic of an urban teaching hospital between august 1999 and december 1999 were analyzed patients were stratified according to the number of clinical features present using the following modified centor criteria history of fever absence of cough presence of pharyngeal exudate and cervical lymphadenopathy the sensitivity of the rapid antigen test was defined as the number of patients with positive rapid antigen test results divided by the number of patients with either positive rapid antigen test results or negative rapid antigen test results and positive throat culture results in the study sample of 498 patient visits the prevalence of gabhs pharyngitis was 28 95 confidence interval ci 24 to 32 the prevalence of gabhs pharyngitis increased as modified centor scores increased 0 or 1 14 2 20 3 43 and 4 52 an increased number of modified centor criteria 0 or 1 2 3 4 was associated with increased rapid antigen test sensitivity 61 76 90 and 97 respectively mantel haenszel trend test p 001 the sensitivity of the rapid antigen test for gabhs is not a fixed value but varies with the spectrum of disease among adults with 3 or 4 clinical criteria for gabhs pharyngitis further study may reveal that culture confirmation of negative rapid antigen test results are not necessary.