What Is a Flu Test?

The Role and Accuracy of Rapid Influenza Tests

We have all been there. You wake up feeling sick. You have a runny nose, your muscles hurt, your throat hurts, you have a headache, and you might have a fever. You may assume it's a bad cold. Or some other upper respiratory infection. But could it actually be the flu?

Unlike the diagnosis of the common cold, there is a test that can help diagnose influenza. It is very fast, non-invasive, but may not always deliver accurate results.

Symptoms

If you are experiencing symptoms of the flu, you may find yourself at the doctor's office at some point. Your health care provider will ask you what symptoms you have and how long you have been sick to determine the next step.

Common flu symptoms include:

  • Fever
  • Fatigue/exhaustion
  • Cough
  • Body aches
  • Headache

If you believe you have the flu, try to see your health care provider within the first 48 hours of the onset of symptoms. If you do need a flu test, it is more likely to be accurate if it is performed within this time frame.

Indications

If flu activity is high in your area and you have many of the symptoms that are common with the illness, your doctor may diagnose and treat you without using a test. Flu tests can be useful but they aren't always necessary to get an influenza diagnosis.

However, flu tests may be especially useful when flu activity is low in your area but your doctor still thinks you might have it. There are many viruses that cause symptoms that are similar to those of the flu.

If your illness isn't caused by the influenza virus, antiviral medicines may not help, so they may not be used in treatment. You are also less likely to develop severe complications from flu-like illnesses than you are from influenza itself, so antiviral medications may not be needed.

Accuracy

Sometimes, your health care provider may decide that you should be tested for the flu. The test may be recommended if your symptoms are unusual or to exclude other community-acquired infections common in your area. Getting the right diagnosis ensures that the most effective treatment is delivered.

Usually, this will involve a nasal or throat culture. Rapid influenza tests can be run in the office; results take about 15 minutes. Some tests are able to tell your health care provider whether you have influenza A or influenza B while others just give a positive or negative result.

Although flu tests can be useful, some are undermined by the high rate of false-negative results. Some rapid tests, like the widely used BD Veritor System, have demonstrated a sensitivity (the ability to make a correct negative diagnosis) of only 78.8%.

The rapid test is far more accurate in infants and becomes less and less accurate the older you get. In people over 70, the test sensitivity may be as low as 60%, all but erasing its benefits.

Other more accurate tests can identify which strain of influenza is circulating in an area (such as influenza A/H1N1, a.k.a. the "swine flu"), although they are used almost exclusively used for research purposes. These tests take longer to run but can help public health officials assess the severity of a flu outbreak, determine the best treatment options, and plan for future influenza vaccines.

Even if your test is negative, if you have significant flu symptoms and flu activity is high in your area, your health care provider may still diagnose you with the flu. Because flu treatment needs to begin soon after the start of symptoms to be beneficial, these specialized tests aren't used to make a diagnosis.

Treatment

Depending on how long you have been sick and your risk for complications, you may be prescribed antiviral medications based on clinical symptoms and risk factors. If you need treatment with antiviral medications, such as Tamiflu, they are most effective if started within the first 48 hours of the onset of symptoms. 

If your doctor decides antiviral medications aren't right for you, you still have other treatment options. You can take over-the-counter medications to lessen the severity of your symptoms. Rest and drinking plenty of fluids is especially important when you have the flu.

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