Strept Cultures and Rapid Strept Tests

Objective. The American Academy of Pediatrics recommends that all negative rapid diagnostic tests for Streptococcus pyogenes pharyngitis be backed up by culture, which creates a dilemma for clinicians who must make treatment decisions without complete diagnostic information at the time of visit. The use of a follow-up serial rapid antigen test instead of a follow-up culture would provide a more timely result.

Methods. Two swabs were collected from children who were suspected of having S pyogenes pharyngitis. Each swab was used for a culture*and an OSOM Ultra Strep A Test rapid antigen test. The gold standard of comparison was defined as the identification ofS pyogenes on either of the 2 culture plates. Three diagnostic strategies were evaluated: a single rapid antigen test, a rapid antigen test with follow-up rapid antigen test (rapid-rapid),and a rapid antigen test with follow-up culture (rapid-culture).

Results. A total of 210 (23.7%) of 887 throat cultures withmatched data were identified with S. pyogenes. A single rapidantigen test had a sensitivity of 87.6% (95% confidence interval[CI]: 83.2%–92.1%), the sensitivity of the rapid-rapid follow-up was 91.4% (95% CI: 87.6%–95.2%), and the sensitivity of the rapid-culture follow-up was 95.7% (95% CI: 93.0%–98.5%),which was significantly higher than the others. As shown in Fig 1, when these test strategies were evaluated on a subgroup with clinical symptoms commonly associated with S pyogenes pharyngitis,the sensitivities all increased and were no longer significantly different. None of the strategies reliably exceeded a 95% sensitivity threshold.

* Both culture plates were incubated at 35°C in 6%–8%CO₂ and were read at 24 and 48 hours. Microbiologists who read the plates were blinded to the rapid test results. Beta-hemolytic streptococci were confirmed as S pyogenes by the BBL Streptocard Acid Latex Test.

Gieseker KE, Roe MH, MacKenzie T, Todd JK. "Evaluating the American Academy of Pediatrics Diagnostic Standard for Streptococcus pyogenes Pharyngitis: Backup Culture Versus Repeat Rapid Antigen Testing."Pediatrics  June 2003;111 (6):e666-e670 [digital article]