STUDY OBJECTIVE: 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.
METHODS: 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.
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).
CONCLUSION: 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.