Testing for influenza, commonly known as the flu, is a standard and frequently necessary part of clinical care, especially during the respiratory illness season. Influenza is a contagious respiratory infection caused by various influenza viruses that can lead to symptoms ranging from mild to severe. Because the flu shares symptoms with other respiratory illnesses, diagnostic testing is often used to confirm the diagnosis and guide appropriate medical decisions. Identifying the specific cause of a patient’s illness is important for both individual treatment and public health monitoring.
Understanding the Different Types of Tests
The primary methods for diagnosing influenza fall into two main categories, each offering a trade-off between speed and accuracy. Both testing types involve collecting a respiratory specimen, usually through a nasal or throat swab.
Rapid Influenza Diagnostic Tests (RIDTs)
The most common point-of-care option is the Rapid Influenza Diagnostic Test (RIDT), which detects viral antigens—proteins on the surface of the flu virus—in a respiratory sample. RIDTs provide results quickly, often within 15 to 30 minutes, allowing for quick clinical decisions. However, traditional RIDTs typically have a lower sensitivity (50% to 70%), meaning they can miss an infection and produce a false negative result.
Molecular Assays
A more precise method is the molecular assay, such as the Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) test. This test is considered the gold standard because it detects the genetic material (RNA) of the flu virus, making it highly accurate with sensitivity often exceeding 90%. While molecular tests offer superior accuracy, they generally require laboratory processing, meaning the turnaround time for results can be longer, sometimes taking several hours to a day or more.
The Role of Timing in Testing
The timing of influenza testing is important because of its direct link to the effectiveness of antiviral medication. These drugs, such as oseltamivir, work best to shorten the duration and severity of the illness when they are started as early as possible. For the greatest clinical benefit, antiviral treatment should ideally be initiated within 48 hours of the onset of symptoms. After this two-day window, the effectiveness of the medication begins to diminish, as the peak of viral shedding has often already passed.
For patients at high risk of complications, such as those who are very young, elderly, pregnant, or have underlying medical conditions, testing and rapid treatment are often prioritized. A confirmed diagnosis is also important for individuals requiring hospitalization or when managing an outbreak in institutional settings. However, due to the tight treatment window, doctors may choose to begin antiviral therapy based on symptoms and local flu activity before test results are confirmed.
Interpreting Results and Test Limitations
Interpreting the results of an influenza test requires considering the test’s limitations and the current level of flu activity in the community. A positive test result generally indicates the presence of the virus, but a false positive can occur, especially when the overall prevalence of flu is very low.
False negatives are a common limitation of the less sensitive rapid tests and can happen if the test is administered too early or too late in the illness when the viral load is low. For this reason, a doctor may still diagnose and treat a patient for the flu based on a strong clinical suspicion, even with a negative rapid test result. In cases where a high-risk patient has a negative rapid test, a more accurate molecular test may be ordered to confirm the true status.