BACKGROUND. Influenza rapid antigen detection (rapid tests) can provide timely identification of infection and aid in clinical decision-making. Although the interpretation of test results depends on test characteristics and influenza prevalence, this information is limited in routine clinical practice.

OBJECTIVE. We sought to assess the times at which rapid tests are most predictive of influenza infection.

METHODS. The New Vaccine Surveillance Network enrolled children aged <5 years who were hospitalized with respiratory symptoms or fever from October 2000 through September 2004. Nasal and throat swabs were obtained, and influenza virus was detected by culture and reverse-transcription polymerase chain reaction. Provider-ordered rapid influenza tests were compared with the criterion standard (culture and reverse-transcription polymerase chain reaction) to determine their sensitivity and specificity. The New Vaccine Surveillance Network also enrolled children in outpatient settings during the 2002–2003 and 2003–2004 influenza seasons and determined the weekly influenza prevalence among symptomatic children. Trends in weekly predictive values of the rapid tests were estimated over the influenza seasons.

RESULTS. Rapid influenza tests had an overall sensitivity of 63% and specificity of 97%. In 2002–2003, the prevalence of influenza in symptomatic outpatient children peaked at 21% and stayed above 10% for ∼4 weeks. In contrast, in 2003–2004, influenza prevalence peaked at 60% and remained above 20% for ∼6 weeks. The positive predictive value of the rapid tests approached 80% when influenza prevalence was ≥15% but decreased to <70% when influenza prevalence was <10%.

CONCLUSIONS. Influenza prevalence varies between and within seasons. On the basis of our estimates, rapid tests are of limited use when prevalence is <10%. The appropriate interpretation of rapid influenza tests requires local influenza surveillance and timely communication of this information to the practitioners.

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