for clinical management. Clinicians should understand that negative results of influenza testing do not exclude influenza virus infection, especially if the time from illness onset to collection of respiratory specimens is more than 5 to 7 days, or if upper respiratory specimens were tested and the patient has lower respiratory tract disease. If influenza is suspected, testing of clinical specimens collected from different respiratory sites can be done (e.g. upper and lower respiratory tract) and can be collected on more than one day to increase likelihood of influenza detection; intubated patients should have endotracheal aspirate specimens tested if influenza is suspected, but not yet confirmed. Detection of influenza virus infection and prompt implementation of control measures is critical to prevention of nosocomial influenza outbreaks. When there is influenza activity in the community, clinicians should consider influenza testing, including viral culture, for patients who develop signs and symptoms of influenza while they are in a health care facility. This should be done as part of a broader surveillance strategy for influenza as discussed in Prevention Strategies for Seasonal Influenza in Heath Care Settings .) Suspected influenza institutional outbreaks For suspected influenza outbreaks in institutions, respiratory specimens should be collected from patients with suspected influenza as early as possible once the outbreak is suspected (See Figure 2 ). If RIDTs are used in these settings, clinical specimens should also be sent for influenza testing by viral culture and RT-PCR to provide detailed information on specific influenza A virus subtype and strains, and