The purpose of this study is to:

1. Evaluate the clinical performance of a newly available highly-sensitive rapid influenza test, Xpert Flu.
2. Derive and validate an adult clinical decision guideline to guide clinical testing of influenza patients who meed CDC criteria for antiviral treatment.
3. Evaluate impact of rapid influenza testing for antiviral treatment.
4. Determine cost effectiveness of influenza testing and treatment strategies.

Four strategically selected medical centers across the US (representing the West Coast, Southwest, Midwest; and East Coast) will enroll adult subjects presenting to the ED with an acute respiratory illness who fulfill CDC recommended criteria for receiving antiviral therapy. At each of the 4 sites, dedicated research coordinators will approach all adult (age > 18 years old) ED patients with broadly defined respiratory related complaints and screen for those who meet CDC recommended criteria for antiviral therapy (i.e. those at 'high-risk for complications' or those with 'potential influenza-related complications).

Consenting subjects will have a nasopharyngeal (NP) swab collected for both Xpert Flu testing and gold standard reverse transcription-PCR (rt-PCR) testing under a research protocol. The investigators will compare the test results from the rapid flu testing to that of the gold standard rt-PCR testing in order to assess the clinical performance characteristics of the new rapid test (Objective 1). The investigators will also collect demographics and medical historical information using a brief structured data form, which will be used in combination with rt-PCR test results to derive and validate a clinical decision guideline (Objective 2).

The investigators will simultaneously randomize 2 of the 4 hospitals to the Rapid Testing Group (which will have systematic rapid flu testing performed with results provided to the clinicians), or the Control Group (which will follow standard of care practice with testing left to the discretion of individual clinicians). This will permit comparison of rates of ED antiviral administration or prescription between the two groups to assess the clinical impact of influenza testing (Objective 3). Finally, cost data from the ED visits and subsequent hospitalization will be collected from subjects in both the rapid testing group and the control group to inform a cost-utility analysis of rapid influenza testing compared to current standard of care (Objective 4).

Inclusion Criteria:

• 18 years old or older
• Acute respiratory tract infection, i.e. in the past two days the subject has experienced any of the following symptoms: new or increased cough, new or increased shortness of breath, change in sputum production (for adults 65 years or older), new sinus pain, new nasal congestion/rhinorrhea, new sore throat, new fever, triage temperature greater than or equal to 38F
• CDC Criteria for influenza antiviral treatment, i.e. is the subject at risk for influenza complications or does the subject have potential current influenza complications including: 65 years old or older, pregnant (or less than 2 weeks post-partum), American Indian or Alaska Native, morbid obesity (BMI greater than 40), current resident of a chronic care facility, chronic pulmonary disease, cardiovascular disease (except isolated hypertension), renal disease, hepatic disease, hematologic disease, metabolic disease, neurologic disease, immunosupression due to [HIV or AIDS, transplant, chemotherapy, medications, other - specify]. OR does the subject have potential current influenza complications such as: admitted to the hospital or a diagnosis of pneumonia.

Exclusion Criteria:

• Unable to speak or understand English or Spanish
• Unable to provide consent
• Lack of follow up information (i.e. working telephone number)
• Have an influenza diagnosis in the past 2 weeks (i.e. either given antivirals or had a confirmatory influenza test)
• Previously enrolled in this study