Doctors can diagnose influenza (flu) using your symptoms alone if many cases of a similar illness have occurred in the community and if the local health department has confirmed a flu outbreak.

Tests to confirm you have the flu and to determine the type of virus may be important if:

• The results may affect treatment decisions. During a confirmed flu outbreak, however, testing may not be needed even if treatment with an antiviral medication is being considered.
• In addition to flu-like symptoms, you have any unusual symptoms that suggest another condition.
• Health authorities have not identified any other cases of flu in your area.

Testing may involve blood tests (rarely used) or a culture to identify the virus. Some cultures take 24 to 48 hours for results, so they will not help your doctor decide whether to prescribe an antiviral medication. A rapid flu test is now available that gives results in 30 minutes. Although this test is not 100% accurate, it can be useful when deciding whether to use an antiviral medication.