Flu tests are diagnostic tools that help confirm the presence of influenza A or B viruses, guiding treatment decisions and limiting the virus’s spread. The most common type is the rapid antigen test, a lateral flow device that delivers results in under 30 minutes. This technology works by detecting specific viral proteins (antigens) in a respiratory sample, providing quick, visual confirmation of infection. Understanding how to read the visual output of these tests and recognize their limitations is important for managing a flu illness.
Interpreting the Test Lines
Reading a rapid flu test involves looking for colored lines that appear in a designated window on the test cassette. These tests typically feature two primary markers: the Control line (C) and the Test line (T). The Control line is a built-in verification that the test ran correctly, confirming the sample fluid migrated across the strip properly.
The Control (C) line must always appear for the test to be successful, regardless of infection status. The Test (T) line appears only if specific viral antigens are present in the sample at a detectable level. A positive result is indicated by the appearance of both the Control line and the Test line. Even a very faint Test line should be interpreted as positive, as it signifies that the test has found the viral proteins.
A negative result shows only the Control (C) line, with the complete absence of any line in the Test (T) region. This outcome suggests that the flu virus was not detected. If the Control line does not appear at all, the result is invalid, even if a Test line is visible. An invalid result usually indicates a technical error, such as improper sample collection or an issue with the test itself, requiring retesting with a new kit.
Factors Affecting Accuracy
Rapid antigen tests have high specificity (95% to 99%), meaning that if the result is positive, it is highly likely the person truly has the flu. False-positive results are uncommon, especially when the flu is circulating widely in the community. However, the sensitivity of these tests is lower, typically falling between 50% and 70%, which means false-negative results are a frequent possibility.
The concentration of the virus in the sample, known as the viral load, directly impacts the test’s ability to detect the infection. If the viral load is low, the test may not generate enough of a signal to create a visible Test line, resulting in a false negative. Timing is another factor, as the virus is most concentrated within the first four days of symptom onset. Testing outside this critical window increases the chance of a false negative result.
Improper sample collection or handling also affects the reliability of the test result. Errors like incorrect swabbing, using the wrong type of swab, or insufficient mixing with the liquid buffer can lead to an inaccurate outcome. Following the manufacturer’s instructions precisely for sample preparation and reading the result within the specified time window, usually 15 to 30 minutes, helps ensure the most accurate interpretation.
Next Steps After Testing
A positive result for influenza A or B should prompt a discussion with a healthcare provider immediately, especially if the person is at high risk for complications. The most important action is to initiate isolation to prevent spreading the contagious virus to others, staying home until the fever is gone and symptoms are improving. Antiviral medications, such as oseltamivir (Tamiflu), are available by prescription and work best when started within one or two days of symptoms beginning.
Contacting a medical provider promptly allows for the assessment of whether antiviral treatment is appropriate, as these drugs can shorten the duration of illness and reduce the risk of serious complications. Even if the 48-hour window has passed, treatment may still be recommended for individuals who are hospitalized or who have underlying health conditions. The provider may offer a virtual visit to prescribe medication, which helps prevent further spread of the virus in a clinical setting.
If a test is negative but flu-like symptoms persist, particularly during a time when the flu is active in the community, a false negative result remains a possibility. In this situation, retesting may be necessary, or a healthcare provider may recommend a more sensitive molecular test, such as a Polymerase Chain Reaction (PCR) test, to confirm the diagnosis. Seeking medical attention is important if symptoms worsen, or if severe symptoms like difficulty breathing or persistent chest pain develop.