Many individuals with the flu are not tested. They either do not seek medical treatment or their healthcare 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 definitive tests take too long to be useful to guide treatment. If it is to be useful in helping healthcare practitioners make decisions about treatment, testing must be done within 3 to 4 days 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 tests—these generally provide results relatively quickly. Some have been approved to be used at the point of care and may be performed in a healthcare practitioner's office or at a hospital patient's bedside. Some types can only detect influenza A; others can detect both A and B but not distinguish between the two. Still others can detect and distinguish between influenza A and B, and some are further able to differentiate between the strains of influenza A, such as H1N1.
- Rapid influenza diagnostic test antigen detection – used to detect influenza antigens in a respiratory sample. These have limited sensitivity and will generally detect 40-70% of influenza cases.
- Molecular tests (e.g., Real-time Reverse Transcription Polymerase Chain Reaction RT-PCR) – these tests detect the influenza viral genetic material (RNA) in a sample. They are generally more sensitive and specific for the influenza virus than rapid antigen tests. They will identify 66% to 100% of influenza cases, depending on the test used.
- Viral culture—the availability of viral cultures is decreasing as most laboratories adopt molecular assays to detect viral infections. Viral cultures are costly and more difficult to perform and take up to 3 to 10 days to provide a result, which makes them less clinically useful for evaluating an individual. 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. Cultures can detect other respiratory viruses that may be causing someone's symptoms.
For more on these, see the article on Influenza Tests.
A healthcare practitioner may order other lab tests in conjunction with an influenza test:
- Strep test—to check for strep throat
- RSV antigen test—to check for respiratory syncytial virus, a virus that often infects young children and the elderly and can mimic influenza
- Sputum culture—to look for bacterial and/or fungal causes of a respiratory infection
- Blood tests—such as a comprehensive metabolic panel (CMP) or complete blood count (CBC) to monitor the health and function of organs such as the lungs and kidneys
- Chest X-ray – sometimes done to evaluate the lungs of a person with symptoms of pneumonia