How the Flu Is Diagnosed

Symptoms of the flu are largely similar to those of the common cold or a respiratory infection, so you may not be able to discern between the issues on your own. Luckily, your doctor can formally diagnose you with the flu with a test that confirms the presence of the influenza virus. However, while fast and non-invasive, a flu test may not always deliver accurate results.

In some cases, your doctor may diagnose you with the flu (or at least recommend you be treated for infection) based on your symptoms, whether or not someone else in your household has the flu, or if cases of influenza are increasing in your area.

Prompt diagnosis of the flu is important, as the infection can have complications, especially for certain individuals. Treatments are available to reduce the duration and intensity of the illness.

how the flu is diagnosed
Ellen Lindner / Verywell

Self-Checks

The main difference between a cold or upper respiratory infection and influenza is how the illnesses come on. While the first two tend to start slowly and gradually worsen over a few days, the flu progresses rapidly, typically starting with a headache that quickly escalates to severe body aches and fatigue.

Common flu symptoms include:

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

If you believe you have the flu, try to see your healthcare 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.

Even if you are convinced that you have the flu, you cannot diagnose yourself. Get a medical opinion to ensure that your self-assessment is accurate and that you are not dealing with another condition that may require treatment.

Physical Examination

Your healthcare provider will ask you what symptoms you have and how long you have been sick to determine next steps. Your doctor will also look in your ears, nose, and throat and listen to your lungs with a stethoscope.

If flu activity is high in your area and your symptoms point to influenza, your doctor may diagnose and treat you simply based on these assessments and an elimination of other causes.

Labs and Tests

There are many viruses that cause symptoms that are similar to those of the flu but will not respond to antiviral medications used to treat influenza.

Given this, your doctor may also perform an in-office flu test to determine if your symptoms are definitely due to the influenza virus. The flu test is especially useful when flu activity is low in your area but your doctor still thinks you might have it.

Rapid influenza tests can be run in the office and takes about 15 minutes to get results. The test typically involves a nasal or throat culture to determine whether influenza A or influenza B is present.

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 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.

Frequently Asked Questions

How does a doctor do a rapid flu test?

They will take a sample of mucus and other fluids from the back of your throat or inside of your nose to be assayed (tested). They may use a cotton swab to gently sweep the back of your throat, which may cause you to gag momentarily, or to swab inside your nose. This could tickle but probably won't be uncomfortable. A doctor also may inject a saline (salt) solution into your nose and then use a suction device to remove the solution.

What illnesses can be mistaken for the flu?

A number of respiratory infections have similar symptoms as the flu, chief among them the common cold and Covid-19. Symptoms of the flu tend to come on more quickly than those of the other illnesses, but otherwise the main differences are the severity of symptoms and the likelihood of complications.

How long are you contagious after you get diagnosed with the flu?

If you're otherwise healthy, you may be able to infect other people with the flu for up to five to seven days after you notice symptoms. Children and people with a weakened immune system may be contagious for longer. Note that people are most contagious during the first three or four days after they become infected with the influenza virus, so they may be contagious before they know they're sick.

When can I go back to my regular activities after having the flu?

The Centers for Disease Control and Prevention advises that you can return to work, school, and other activities after having the flu if:

  • You have been fever-free for 24 hours without taking a fever-reducing medication, such as Tylenol (acetaminophen), or
  • It has been four or five days since your symptoms started, and they have largely resolved

A Word From Verywell

Even if your test is negative, if you have significant flu symptoms and flu activity is high in your area, your healthcare provider may still diagnose you with the flu to be on the safe side. Antiviral medications such as Tamiflu are most effective if started within the first 48 hours of the onset of symptoms.

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