Performance of influenza rapid antigen testing in influenza in emergency department patients.

BACKGROUND The use of rapid antigen tests to triage specimens for polymerase chain reaction (PCR) testing from emergency department patients with influenza-like illness during surveillance for novel influenza viruses has been suggested. OBJECTIVE To measure the observed sensitivity and specificity for a widely used rapid antigen test (Binax) using a PCR-based assay (Medical Diagnostic Laboratories). METHODS Nasopharyngeal samples were taken with flocked swabs (Copan Diagnostics) from patients presenting to the emergency department of a community hospital. Samples were analysed using a rapid antigen and a PCR-based test. PCR testing was used as the criterion reference. Sensitivity and specificity were calculated for influenza and influenza A. Positive predictive values were calculated over a range of possible prevalence. RESULTS Samples from 566 unique patients were tested using both methods. Sensitivity was 69.1% (95% CI 58.9% to 78.1%) and specificity was 97.7% (95% CI 95.8% to 98.8%) for the detection of any influenza and 75.3% (95% CI 64.7% to 84.0%) and 97.8% (95% CI 95.9% to 98.9%), respectively, for influenza A only. The resultant positive predictive value ranges from 23% to 77% when the prevalence ranges from 1% to 10%. CONCLUSION When planning early outbreak surveillance, provision of adequate PCR testing capacity rather than triaging specimens using rapid antigen testing for influenza is advisable.