Many individuals with the flu are not tested. They either do not seek medical treatment or their health practitioner 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 health practitioners make decisions about treatment, testing must be done within 48 hours of the onset of symptoms.
Testing is primarily performed for those who are seriously ill (hospitalized) and for 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 diagnostic tests (RIDTs) – used to detect influenza A or influenza A and B and to guide antiviral treatment. These can be performed in a doctor's office with results often available in 15 minutes. However, they have limited sensitivity and can result in false negatives and false positives, especially during peak flu activity. They do not provide information about influenza A subtypes.
- Viral culture – considered the traditional "gold standard" for diagnosing influenza but can take 3 to 7 days. Culture can detect influenza A and B and determine the strain and subtype of influenza, which helps formulate the influenza vaccine for the following year and identify emerging strains that could lead to a pandemic. Culture can detect other respiratory viruses that may be causing someone's symptoms.
- Real-time polymerase chain reaction (RT-PCR) – detects viral genetic material and can yield results in several hours. It is the most sensitive influenza test and is highly specific, but false negatives are still possible. It is being used increasingly in clinical settings.
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.