How Is the Flu Diagnosed?

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How is the Flu Diagnosed?. Echo/Cultura/Getty Images

Unlike the diagnosis of the common cold, the flu is often diagnosed based on a combination of symptoms and a flu test.

Flu 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 have several symptoms of the flu and flu activity is high in your area, your health care provider may stop here. Depending on how long you have been sick and your risk for complications, you may be prescribed antiviral medications based on a clinical symptoms and risk factors. If your doctor decides they aren't right for you, you still have several treatment options.

Flu Testing

Sometimes, your health care provider may decide that you should be tested for the flu. Usually, this will involve a nasal or throat culture and the test 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.

While these tests are useful, they are not definitive when making a flu diagnosis because false negatives are common.

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.

Other, more accurate tests can be performed by specialized labs and can determine exactly which strain of influenza is causing the illness.

These tests help public health officials determine what strains of influenza are circulating. They are useful for public health officials because they help them see the severity of a flu outbreak, how best to treat the particular strain and can even help them plan for future influenza vaccines. However, these tests take longer and are not used to diagnose and determine treatment options for individuals.

If you believe you might have the flu, try to see your health care provider within the first 48 hours of the onset of your symptoms. If you do need a flu test, it is more likely to be accurate if it is performed during this time frame. And if you need treatment with antiviral medications, they are most effective if started within the first 48 hours of the onset of symptoms. 


"Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests for the 2010-2011 Influenza Season." Seasonal Influenza (Flu) 22 Dec 10. Centers for Disease Control and Prevention. 30 Jun 11.

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