Q&A

Negative antigen test misses <5% of strep pharyngitis

J Fam Pract. 2003 October;52(10):747-769
By
John Gazewood, MD, MSPH
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  • BACKGROUND: Thirty percent of pharyngitis in children is caused by Streptococcus pyogenes. Since it is the practice in the US to treat this cause of pharyngitis, current recommendations for diagnosis state that a confirmatory culture should be obtained in patients with a negative rapid strep antigen test. This strategy can be cumbersome in clinical practice, may delay treatment, and result in unnecessary use of antibiotics while awaiting culture results.
  • STUDY DESIGN: The authors performed a prospective cohort study. They enrolled 891 children presenting to a large pediatric practice who were suspected of having S pyogenes pharyngitis. Although the authors did not explicitly state inclusion and exclusion criteria, the spectrum of patients is likely representative of children presenting with a sore throat to a family practice.
  • OUTCOMES MEASURED: Sensitivity and specificity of different testing strategies.
  • RESULTS: Four patients did not have paired cultures, leaving 887 patients for analysis. The prevalence of strep throat in the entire sample was 23.7%; among the high-risk patients the prevalence was 40%. The combination of rapid test and culture was statistically more sensitive than other strategies (P=.01), but the negative predictive value for all strategies was >95%.
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PRACTICE RECOMMENDATIONS

For patients with an average risk of having streptococcal pharyngitis, a number of testing strategies can rule out infection, including a single negative rapid strep antigen test, which has a negative predictive value of more than 95%. In patients at higher risk of having strep, based on clinical criteria, all the test strategies being studied are more likely to be falsely negative.

While it is reasonable to rely on a variety of strategies to exclude strep throat in average-risk patients, high-risk patients with a negative rapid antigen test should have an additional rapid strep test or a throat culture to exclude strep with a certainty of greater than 95%.

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