Background: Nursing home influenza outbreaks occur in spite of established vaccination programs, and require rapid and sensitive laboratory confirmation for timely intervention.
Objectives: To evaluate diagnostic approaches for rapid confirmation of nursing home influenza outbreaks.
Study design: Influenza virus real-time PCR and Directigen Flu A+B enzyme immunoassay were performed on nasopharyngeal swabs, nasopharyngeal washes and throat swabs collected from residents with clinical suspicion of influenza during seven probable nursing home outbreaks in 2004-2005 and 2005-2006. The efficacy of specimen sampling and transport management by Public Health Service outbreak team was evaluated.
Results: PCR detected influenza RNA in 80% (68/85) of specimens from 81% (38/47) residents, confirming six suspected outbreaks. Immunoassay sensitivity was highest on nasopharyngeal swabs (38%; 11/29) with a positive predictive value of 100% compared to PCR. Nasopharyngeal swabs were equally sensitive to nasopharyngeal washes by PCR. Nasopharyngeal wash sampling appeared unpractical due to common underlying disability of residents. Outbreak team support was associated with a shorter time to PCR diagnosis compared to outbreaks with no logistical support (mean, 28.2h vs. 84h; P=0.05).
Conclusions: Influenza real-time PCR on nasopharyngeal swabs, obtained by Public Health Service outbreak teams, enabled rapid and sensitive confirmation of nursing home influenza outbreaks.