Influenza tests are performed to diagnose the flu, to distinguish it from other viral infections or other causes of symptoms, to guide treatment, and to monitor the spread of influenza through communities and throughout the world.
Many individuals with the flu are not tested. They either do not seek medical treatment or their doctor makes the diagnosis of probable-flu based upon the person's symptoms, the fact that it is the flu season, and based upon the presence of influenza in the community. This is partially because rapid influenza tests will not detect every case of influenza and partially because more sensitive tests take too long to be useful to guide treatment. If it is to be useful in helping doctors make decisions about treatment, testing must be done within 48 hours of the onset of symptoms.
When rapid test results are negative, doctors may order follow-up testing but they will not wait for the results to initiate treatment. Testing is primarily performed on those who are seriously ill (hospitalized) and on those who are at risk for a severe case of influenza or for complications.
Several different types of influenza tests may be used to detect an infection. These include:
- Rapid influenza antigen test – used to detect influenza A or influenza A and B and to guide antiviral treatment. Can be performed in a doctor's office with results often available in 30 minutes.
- Direct fluorescent antibody stain (DFA) – more sensitive than rapid testing but requires specialized training to interpret.
- Viral culture – considered the gold standard for diagnosing influenza but can take 3 to 7 days. Can detect influenza A and B and determine the strain of influenza.
- Real-time polymerase chain reaction (RT-PCR) – a sensitive test that detects viral genetic material. A specific version of this test has been developed to detect the 2009 H1N1 influenza virus.
- Influenza A or B antibody test – blood tests performed to detect the body's immune response to an influenza infection. These tests are typically done for research or public health purposes.
For more on these, see the article Influenza Tests.
- Chest X-ray - sometimes done to evaluate the lungs of a person with symptoms of pneumonia.