Failure of routine diagnostic methods to detect influenza in hospitalized older adults

To define the utility of using routine diagnostic methods to detect influenza in older, hospitalized adults.

Descriptive study.

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.

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.

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%).

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.