A flu test is designed to detect the presence of specific proteins or genetic material from the influenza A or B viruses in a person’s respiratory sample. A false positive result occurs when a test indicates that the influenza virus is present, but the person is not infected with the flu. These inaccurate results can be misleading for both patients and healthcare providers. They potentially lead to unnecessary treatment or isolation. The causes for a false positive stem from the test technology itself, biological interference, or mistakes in the collection and processing steps.
Inherent Limits of Rapid Antigen Tests
The reliability of a flu test is heavily dependent on the type of technology used, with Rapid Antigen Detection Tests (RADTs) being the most common source of false positive results. RADTs are quick, typically delivering results within 15 minutes, because they look for surface proteins, or antigens, of the virus. These tests rely on an immunoassay where antibodies in the test kit attempt to bind to the influenza antigens in the patient sample. Molecular tests, such as Polymerase Chain Reaction (PCR), are considered the gold standard because they amplify and detect the virus’s genetic material, offering much higher specificity. While RADTs have improved, their specificity is still lower than PCR tests, making them more susceptible to false readings. The likelihood of a positive result being false is particularly high when the overall prevalence of the flu in the community is low, such as during the summer months.
Viral Interference (Cross-Reactivity)
One biological reason for a false positive result is a phenomenon known as cross-reactivity, or viral interference. This occurs when the influenza test’s antibodies mistakenly recognize a protein from an entirely different virus. The antigen-based flu test is designed to bind to a specific molecular shape on the influenza virus. However, other circulating respiratory viruses may have surface proteins that are structurally similar enough to trick the test’s detection antibodies. Several common respiratory pathogens have been implicated in causing this kind of interference. Certain seasonal human coronaviruses, adenoviruses, and parainfluenza viruses can possess antigens that share sufficient structural homology with influenza A or B proteins to elicit a positive signal on the test. This biological mechanism explains why a person can be genuinely sick with a respiratory illness and still receive a flu false positive.
Errors in Sample Collection and Processing
Procedural errors during the collection and processing of the sample can also lead to inaccurate results. The quality of the sample collected is paramount, and improper swabbing technique can inadvertently introduce contaminants or interfere with the test chemistry. For example, using a swab that was not supplied with the test kit, or using one that contains materials that chemically interfere with the assay, can lead to non-specific positive readings.
Operational factors within the clinical setting also pose a risk for false positives. The use of expired reagents, which may have degraded over time, can compromise the specificity of the antibody-antigen binding reaction. Contamination is another concern, where a sample can be inadvertently exposed to a strong concentration of influenza virus, such as from a previously tested positive sample in the same work area. Finally, with older, manual rapid tests, incorrect reading or interpretation of the test line—especially in cases where a faint line appears—can mistakenly be reported as a positive result.