Diagnosis for Flu

Signs and symptoms to look out for

Your respiratory illness might be flu if you have signs or symptoms, including fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and/or fatigue. Some people may have vomiting and diarrhea, though this is more common in children. People may be sick with flu and have respiratory symptoms without a fever. Influenza viruses usually cause illness during the colder months of the year. However, flu can also occur outside of the typical flu season. In addition, other viruses can also cause respiratory illness similar to flu. So, it is impossible to tell for sure if you have flu based on symptoms alone. If your health care provider needs to know whether you are sick with flu, there are laboratory tests that can be done.

Different kinds of flu tests

A number of tests are available to detect influenza viruses in respiratory specimens. The most common are called "rapid influenza diagnostic tests (RIDTs)." RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response. Some RIDTs are FDA approved and available for home use, including tests that can detect influenza A and B viruses, and SARS-CoV-2 (the virus that causes COVID-19) in upper respiratory tract specimens. These tests can provide results within approximately 10-15 minutes but may not be as accurate as other flu tests. Therefore, you could still have flu, even though your rapid test result is negative. Other flu tests called "rapid molecular assays" detect genetic material of the influenza virus. Rapid molecular assays produce results in 15-20 minutes and are more accurate than RIDTs.

In addition to RIDTs and rapid molecular assays, there are several more accurate flu tests available that must be performed in specialized laboratories, such as hospital and public health laboratories. These tests include reverse transcription polymerase chain reaction (RT-PCR), and immunofluorescence assays. All of these tests require that a health care provider swipe the inside of your nose or the back of your throat with a swab and then send the swab for testing. Once at the laboratory, results can take one to several hours.

At-home flu tests

CDC does not have specific guidance for the use of at-home flu tests. CDC recommends people at higher risk of serious flu complications seek medical care promptly for flu-like symptoms to determine if they need antiviral medications.

If you test positive for flu using one of these home-based tests and are at higher risk of serious flu complications, contact your health care provider as soon as possible to ask about treatment with flu antiviral drugs. More information on CDC's flu treatment recommendations is available at Treating Flu with Antiviral Drugs | Influenza (Flu) | CDC.

For a list of flu diagnostic tests, please visit: Influenza Diagnostic Tests | FDA

Flu detection for rapid tests

During a flu outbreak, a positive rapid flu test likely means you have flu. However, a negative rapid flu test does not necessarily mean you do not have flu. Rapid tests vary in their ability to detect influenza viruses, depending on the type of rapid test used and on the type of influenza viruses circulating. Also, rapid tests appear to be better at detecting influenza viruses in children than in adults. This variation in ability to detect viruses can result in some negative rapid test results in people who really have flu. This situation is called a false negative test result. Despite a negative rapid test result, your health care provider may diagnose you with flu based on your symptoms and their clinical judgment.

Health care providers and testing

While your health care provider may test you for flu, not everyone who goes to the health care provider with flu-like symptoms will be tested. After evaluating you, your health care provider may choose to diagnose you with flu without the need for testing based on your symptoms and their own clinical judgement.

Flu co-infections

It is possible to have flu as well as other respiratory illnesses including COVID-19 at the same time.

Detecting both flu and other respiratory viruses

Symptoms for some respiratory viruses overlap (e.g., flu and COVID-19), and so you may be tested for one more than one virus. There are tests that check for seasonal influenza viruses and SARS-CoV-2, the virus that causes COVID-19. Testing for these viruses at the same time helps health care providers diagnose and treat appropriately, and the results can also give public health officials important information about how flu and COVID-19 are spreading and what prevention steps should be taken. The tests also help public health laboratories save time and testing materials, and possibly to return test results faster by testing for multiple pathogens simultaneously.