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Rapid antigen detection testing in diagnosing group A beta-hemolytic streptococcal pharyngitis
S A Joslyn1, G L Hoekstra, J E Sutherland
1School of Health Service, University of Northern Iowa, Cedar Falls 50614-0241, USA.
Background:
The purpose of this study was to determine the accuracy of diagnosing group A beta-hemolytic streptococci (GABHS) with rapid antigen testing compared with throat culture methods commonly used.
Methods:
Two separate studies were conducted. Initially, 182 patients with acute pharyngitis had both throat culture (sheep blood agar or Strep Select Agar) and rapid antigen detection screening tests (Directigen1-2-3 Group A Strep) performed. For the second study, a rapid antigen detection test (Directigen1-2-3 Group A Strep) was obtained from 614 patients. All subjects who screened negative (n = 469) received a throat culture. All subjects who screened positive (n = 145) were treated with antibiotics, and 31 of these patients received a throat culture. Statistical analyses included calculating sensitivity, specificity, positive and negative predictive values, and prevalence.
Results:
For the initial 182 patients, the prevalence of GABHS was 12 percent. Sensitivity was 95.45 percent, specificity was 96.25 percent, positive predictive value was 77.78 percent, and negative predictive value was 99.35 percent. In the second group of subjects, four false-negatives were present (negative predictive value = 99.18 percent).
Conclusions:
Results of these pilot studies indicate that an extremely low percentage (< 1 percent) of subjects with GABHS escaped detection with our rapid screening test methods. These results conflict with results from previous investigations, which have reported relatively low specificity and sensitivity of rapid antigen detection tests when compared with throat cultures. Results from this study support treatment protocols based on a rapid screening test as a single diagnostic test.