Introduction: Who Should Undergo Diagnostics and When to Seek Them

Influenza, commonly known as the flu, is a contagious respiratory illness that affects millions of people every year. While many people recover on their own within a week, knowing when to seek diagnostic testing is important for your health and the health of those around you. Diagnostics are medical tests that help healthcare professionals determine whether you have the flu virus, which type it is, and how severe your infection might be.

Most people who develop flu-like symptoms do not need to visit a doctor or undergo testing. If you are otherwise healthy and experience typical flu symptoms such as fever, cough, body aches, and fatigue, staying home and resting is usually sufficient. However, certain groups of people should consider getting tested and seeking medical care as soon as symptoms appear.^[1]

You should seek diagnostic testing and medical attention if you fall into a higher-risk category. This includes people aged 65 years and older, young children (especially those under 5 years old, and particularly those under 2), pregnant women, and people with chronic medical conditions such as asthma, diabetes, heart disease, chronic lung disease, or weakened immune systems. People with obesity (a body mass index or BMI greater than 30), those living in long-term care facilities, and children under 19 years who regularly take aspirin are also at increased risk for serious flu complications.^[3][11]

Early diagnosis is especially important for these high-risk groups because antiviral medications work best when started within the first two days of symptom onset. These medications can shorten the duration of illness, make symptoms milder, and help prevent serious complications like pneumonia. For this reason, if you are at higher risk and develop flu symptoms, you should contact your healthcare provider right away rather than waiting to see if you feel better on your own.^[9]

It’s also advisable to seek diagnostics if your flu symptoms do not improve after several days or if they seem to get worse over time. Sometimes the flu can lead to secondary infections or complications that require different treatment. Additionally, during times when both flu and other respiratory illnesses like COVID-19 are circulating in the community, testing can help determine which virus you have, as this affects treatment decisions.^[11]

Diagnostic Methods for Identifying Influenza

When you visit a healthcare provider with flu-like symptoms, they will first perform a physical examination and ask about your symptoms, when they started, and whether you’ve been in contact with anyone who has been sick. Sometimes, especially during flu season when influenza is widespread in the community, your doctor may diagnose flu based on your symptoms and the physical exam alone, without ordering any tests. This is called a clinical diagnosis, which means the doctor makes a decision based on what they observe and what you tell them about how you feel.^[10]

However, in many situations, particularly if you are at high risk for complications or if the diagnosis is uncertain, your healthcare provider may order a flu test. These tests can confirm whether you have influenza, identify which type of flu virus is causing your illness, and rule out other respiratory infections that may require different treatments.^[10]

Types of Flu Tests

There are several types of diagnostic tests available to detect influenza viruses. Each has different characteristics in terms of how quickly results are available, how accurate they are, and where they can be performed.

Rapid influenza diagnostic tests (RIDTs), also called rapid antigen tests, are among the most commonly used flu tests. These tests look for proteins called antigens on the surface of the influenza virus. Antigens are parts of the virus that trigger your immune system to respond. To perform this test, a healthcare provider typically uses a swab to collect a sample from your nose or throat. The sample is then placed in a testing device that can provide results within 10 to 30 minutes, sometimes even faster.^[10]

The main advantage of rapid antigen tests is their speed, which allows doctors to make treatment decisions during your visit. However, these tests are not as accurate as other methods. They are better at ruling out the flu when results are negative than at confirming it when results are positive. This means that if your rapid test is negative but your doctor still suspects you have the flu based on your symptoms and the local flu activity, they may treat you as if you have the flu anyway or order a more accurate test.^[10]

Molecular tests, also called nucleic acid amplification tests (NAATs) or polymerase chain reaction (PCR) tests, are the most accurate type of flu diagnostic test. These tests detect the genetic material of the influenza virus rather than just surface proteins. Because they look for the virus’s genetic code, they can identify even small amounts of virus in your sample, making them highly sensitive and specific. PCR tests can also distinguish between different types and subtypes of influenza viruses, such as influenza A versus influenza B, and even specific strains like H1N1 or H3N2.^[10]

For a molecular test, a healthcare provider collects a sample from your nose or throat using a swab, similar to the rapid test. However, the sample must be sent to a laboratory for analysis, which typically takes several hours to a few days depending on the laboratory’s location and workload. Some newer PCR tests can provide results more quickly, within one to two hours. Despite taking longer, molecular tests are preferred when accurate diagnosis is crucial, such as for hospitalized patients, those at high risk for complications, or when public health surveillance requires precise identification of circulating flu strains.^[10]

It’s worth noting that some molecular tests can simultaneously detect multiple respiratory viruses, including both influenza and COVID-19. These combination tests are particularly useful during seasons when several respiratory illnesses are circulating, as they help doctors determine which virus is causing your symptoms and guide appropriate treatment.^[10][11]

A third category of flu tests includes direct immunofluorescence tests. These tests use antibodies that have been tagged with fluorescent dye to detect flu virus antigens in respiratory samples. The sample is examined under a special microscope that causes the fluorescent dye to glow if flu virus is present. These tests are more accurate than rapid antigen tests but less accurate than molecular tests. They typically take a few hours to complete and require specialized laboratory equipment and trained technicians.^[10]

Sample Collection

To perform any of these flu tests, healthcare providers need to collect a sample from your respiratory tract. The most common method is a nasopharyngeal swab, where a long, thin swab is inserted through your nostril to the back of your nose and throat area. This can be uncomfortable but only takes a few seconds. Alternatively, providers may use a nasal swab that doesn’t go as deep, a throat swab, or ask you to provide a nasal wash or aspirate sample. The type of sample collected can affect test accuracy, with nasopharyngeal swabs generally considered the most reliable.^[10]

At-Home Testing

Recently, at-home flu tests have become available for people aged 2 years and older. These tests work similarly to rapid antigen tests but can be performed at home without visiting a healthcare facility. If you use an at-home test, it’s important to inform your healthcare provider of the result, whether positive or negative. Your provider may recommend confirming the result with a laboratory test, especially if the result doesn’t match your symptoms or if you are at high risk for complications.^[10]

When Testing Is Recommended

Testing for flu is particularly recommended during flu season when influenza is actively spreading in your community. Your healthcare provider is more likely to order a flu test if you are hospitalized, if you are at high risk for flu complications, if there is uncertainty about whether your symptoms are caused by flu or another illness, or if test results would change treatment decisions. During times when flu activity is low, testing may not be necessary unless there are specific reasons to suspect influenza.^[10]

Diagnostics for Clinical Trial Qualification

When patients with influenza are being considered for participation in clinical trials, more rigorous and standardized diagnostic procedures are typically required. Clinical trials are research studies that test new treatments, vaccines, or diagnostic methods to determine whether they are safe and effective. To ensure that trial results are accurate and reliable, researchers need to be absolutely certain that participants actually have the disease being studied.

In the context of influenza clinical trials, molecular tests such as PCR are almost always used as the standard diagnostic criterion for enrolling patients. These tests are preferred because of their high accuracy in detecting and identifying influenza viruses. Unlike rapid antigen tests, which might miss some cases of flu or give false positive results, PCR tests can detect very small amounts of viral genetic material and can distinguish between different types and subtypes of influenza with great precision.^[10]

Clinical trial protocols typically specify exactly which diagnostic test must be used, what type of sample must be collected, and how quickly after symptom onset the sample must be taken. For example, a trial testing a new antiviral drug might require that participants have their flu confirmed by PCR testing within 48 hours of symptom onset, since antiviral medications work best when started early. The trial might also specify that participants must have influenza type A or type B, or even a specific strain, depending on what the treatment is designed to target.

In addition to confirming the presence of influenza virus, clinical trials often use diagnostic tests to monitor how well treatments are working. This might include repeated PCR testing to measure how quickly the virus disappears from respiratory samples after treatment begins. Some trials also measure viral load, which is the amount of virus present in a sample. A decreasing viral load over time suggests that the treatment is successfully fighting the infection.

Blood tests may also be used in flu clinical trials to measure immune responses. For example, researchers might test for antibodies against the flu virus before and after vaccination in a vaccine trial, or they might measure markers of inflammation to understand how the body responds to infection and treatment. These additional tests help researchers understand not just whether a treatment works, but how and why it works.

Clinical trials may also require chest X-rays or other imaging tests if the study involves patients with flu complications such as pneumonia. These imaging tests help researchers document the severity of illness and track improvement over time. Blood tests measuring oxygen levels, kidney and liver function, and other indicators of overall health are often required to ensure patient safety throughout the trial.