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 approximately 4 weeks. In contrast, in 2003-2004, influenza prevalence peaked at 60% and remained above 20% for approximately 6 weeks. The positive predictive value of the rapid tests approached 80% when influenza prevalence was > or = 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.