Objective: To define the utility of using routine diagnostic methods to detect influenza in older, hospitalized adults.
Design: Descriptive study.
Setting: One academic hospital and 1 community hospital during the 2006-2007 and 2007-2008 influenza seasons. Participants. Hospitalized adults 50 years of age or older.
Methods: Adults who were 50 years of age or older and hospitalized with symptoms of respiratory illness were enrolled and tested for influenza by use of reverse-transcriptase polymerase chain reaction (RT-PCR). Using RT-PCR as the gold standard, we assessed the performances of rapid antigen tests and conventional influenza culture and the diagnostic use of the clinical definition of influenza-like illness.
Results: Influenza was detected by use of RT-PCR in 26 (11%) of 228 patients enrolled in our study. The sensitivity of the rapid antigen test performed at bedside by research staff members was 19.2% (95% confidence interval, 8.51%-37.9%); the sensitivity of conventional influenza culture was 34.6% (95% confidence interval, 19.4%-53.8%). The ability to detect influenza with both the rapid antigen test and culture was associated with patients with a higher viral load (P=.002 and P=.001, respectively). The ability to diagnose influenza by use of the clinical definition of influenza-like illness had a higher sensitivity (80.8%) but lacked specificity (40.6%).
Conclusion: Because rapid antigen testing and viral culture have poor sensitivity (19.2% and 34.6%, respectively), neither testing method is sufficient to use to determine what type of isolation procedures to implement in a hospital setting.