Diagnosis

• whether it is flu season,
• whether there is flu in the community, and
• whether the person's complaints fit the current pattern of flu symptoms.

Studies indicate that, when adults are diagnosed with influenza based upon fever and cough, the diagnosis correlates with viral cultures between 55% and 78% of the time.

While not all patients require lab tests to verify influenza, some populations benefit from them and some circumstances require them. Diagnostic tests assist the clinician to differentiate between influenza and a number of respiratory diseases which mimic it. Early detection and identification of influenza can maximize infection control, encourage vaccination, guide selection of antiviral medications and reduce the inappropriate use of antibiotics.

Diagnostic tests for influenza should be done within 1-4 days of illness. These tests include:

1. Viral culture: Cultures are collected at sentinel sites around the world to determine which subtypes are posing the greatest risk of epidemic. Vaccines are produced based upon the data collected. Results from viral cultures are usually available within 5-10 days. Specimens may be collected by nasopharyngeal swab, nasal or bronchial wash, nasal aspirate or sputum.
2. Direct Fluorescent Antibody Staining: This test can determine if flu is present, and can distinguish between Influenza A or B in 2-4 hours. The short result time can be useful in the clinical decision to begin antiviral therapy. Specimens may be collected by nasopharyngeal swab, nasal or bronchial wash, nasal aspirate or sputum.
3. Reverse Transcriptase Polymerase Chain Reaction: Determines if flu is present, and can distinguish between Influenza A or B in 2-4 hours. Specimens may be collected by nasopharyngeal swab, nasal or bronchial wash, nasal aspirate or sputum.
4. Serology can determine if flu virus is present, and distinguish between A vs. B, but requires paired acute and convalescent serum samples and about 2 weeks for results.
5. Enzyme Immuno Assay which can determine if flu virus is present, and distinguish between A vs. B in 2 hours. Specimens may be collected by nasopharyngeal swab, nasal or bronchial wash, nasal aspirate or sputum.
6. Rapid Diagnostic Tests are able to return a result, either positive or negative, for influenza in as few as 10 minutes. These tests have the advantage of providing results at the point of care and can support the decision to initiate antiviral therapy. However, not all tests are able to differentiate between influenza A or B. There are also variations in specimen collection requirements. The rapid tests may also be followed up with a viral culture to verify and identify as part of a formal surveillance program. Finally, vaccination with Live Attenuated Influenza Vaccine, within a week prior to rapid testing, can cause a false positive result.

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