How Is the Flu Diagnosed?

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? Contrary to popular belief, the flu does not usually cause vomiting and diarrhea. It is a respiratory virus.

Unlike the diagnosis of the common cold, the flu is often diagnosed based on a combination of symptoms and a flu test. Do you know when you should go to the doctor to be diagnosed?

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 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.


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.

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 (such as H1N1) is causing the illness during the season. These tests help researchers determine which strains of influenza are circulating in a given area. 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 help them plan for future influenza vaccines. However, these tests take longer to run and are not used to diagnose and determine treatment options for individuals.

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.


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, 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 several treatment options. You can take over the counter medicines to relieve your symptoms temporarily. Rest and drinking plenty of fluids is especially important when you have the flu.

A Word From Verywell

As you can see, rapid flu tests are useful in diagnosing the flu, but aren't always necessary. Talk to your healthcare provider to decide if you need one when you are sick. 

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