Failure of Routine Diagnostic Methods to Detect Influenza in Hospitalized Older Adults

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Abstract
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 andP= .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.