Flu: Diagnosis

Written by the Healthline Editorial Team | Published on July 28, 2014
Medically Reviewed by George Krucik, MD, MBA on July 28, 2014

Flu: Diagnosis

A doctor is usually able to diagnose a case of influenza by simply observing common symptoms and noting the presence of influenza in the local community. Most people will not require tests for diagnosis.

However, laboratory tests are sometimes necessary for certain high-risk individuals, such as children younger than five, adults over the age of 65, and pregnant women. These tests can help the doctor determine the specific strain of flu present in an individual. They can also help the doctor verify that the illness is not caused by another pathogen that produces similar symptoms. Influenza’s symptoms are similar to other respiratory infections including, but not limited to, parainfluenza viruses, respiratory syncytial virus (RSV), and adenoviruses.

Laboratory tests may be helpful in determining treatment options for the patient as well. They can also help the doctor to establish whether influenza is the cause of an outbreak in a closed setting such as a hospital, nursing home, or summer camp.

Testing for the Flu

Most influenza tests require a nasal or throat swab to determine the type of influenza virus. Occasionally, a blood sample is required for antibody testing. A variety of tests can determine the type of influenza virus in less than 30 minutes. However, these tests are not as accurate as those that take longer to obtain results. The following tests are those most commonly used in influenza testing.

Rapid Influenza Diagnostic Test (RIDT)

RIDT is a test that can quickly determine whether a patient’s symptoms are caused by the flu virus or by another viral or bacterial infection that requires more serious treatment. A number of rapid tests are available, and they all require a simple nasal swab. The results are usually available on the same day. Rapid tests are most accurate when administered within the first 48 hours after symptoms appear.

Some rapid tests can distinguish between flu virus types A and B, but none of them can identify subtypes. Additionally, the American Association for Clinical Chemistry (AACC) notes that the tradeoff for the speed of these tests is that they can miss up to 30 percent of flu infections.

Direct Fluorescent Antibody Stain (DFA)

This test uses a nasal swab to determine the presence or absence of antibodies to the influenza virus in nasal secretions. The test can tell the difference between Type A and Type B virus. However, it is unable to distinguish between subtypes of Type A viruses like H1N1 or H3N2.

Influenza Virus Culture Test

The virus culture test is the most accurate test for influenza and can determine both virus type and subtype. For this test, a nasal swab is taken. Then, a culture of the virus is grown in a laboratory. Results can take three to 10 days.

Sometimes a culture test is taken to confirm a rapid flu test. It can also find other respiratory viruses as well.

This type of influenza test also helps healthcare workers determine the dominant strain of virus circulating in a community or find out whether unusual cases indicate that a new strain is present. Information from these culture tests is also used to formulate the next year’s flu vaccine.

Other Tests

If secondary infections like pneumonia or other complications are suspected, your physician may order other tests or procedures. These may include chest X-rays, blood tests, or sputum cultures. A sputum culture is used to test substances from the lungs and bronchi (the tubes that carry air to the lungs) to determine whether an infection is present.  

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