Should clinical case definitions of influenza in hospitalized older adults include fever?

Introduction Influenza is a major cause of morbidity and mortality in elderly persons. Fever is included in all standard definitions of influenza-like illness (ILI), yet older patients may have diminished febrile response to infection. Therefore, we examined the utility of various thresholds to define fever for case definitions of influenza in persons 65years of age. Methods Data from two prospective surveillance studies for respiratory viral infection in adults hospitalized with acute cardiopulmonary illnesses with or without fever were examined. The highest temperature reported prior to admission or measured during the first 24h after admission was recorded. The diagnosis of influenza was made by a combination of viral culture, reverse-transcription polymerase chain reaction, antigen testing, and serology. Results A total of 2410 subjects (66% 65years of age) were enrolled; 281 had influenza (261 influenza A, 19 influenza B, and one mixed influenza A and B). The commonly used definition of ILI (fever 378 degrees C and cough) resulted in 57% sensitivity and 71% specificity in older adults. Receiver operating characteristic curves examining the various temperature thresholds combined with cough and/or sore throat showed the optimal balance between sensitivity and specificity to be 379 degrees C (AUC 071) and 373 degrees C (AUC 066), in younger and older persons, respectively. Conclusion Clinical decision rules using the presence of cough and fever may be helpful when screening for influenza or empiric antiviral treatment when rapid influenza testing is not available; however, lower fever thresholds may be considered for elderly subjects.