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Symptoms and disgnosis of flu, and what to do if you get sick
Health  |  Fri - March 21, 2025 11:38 am  |  Article Hits:23  |  A+ | a-

Diagnosis for Flu

Centers for Disease Control and Prevention (CDC)


CDC

KEY POINTS

  • Flu is a contagious respiratory illness that can cause fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and/or fatigue.
  • There are multiple tests that can detect flu viruses.
  • Although testing is the only way to know if you have flu, your doctor may diagnose you based on your symptoms.

Symptoms to look out for

Your respiratory illness might be influenza (flu) if you have 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. Flu viruses usually cause the most 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 doctor needs to know for sure 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 flu 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. 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 flu 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), viral culture, 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. Results may take one to several hours.

Flu detection for rapid tests

During a flu outbreak, a positive rapid flu test is likely to indicate flu virus infection. However, rapid tests vary in their ability to detect flu viruses, depending on the type of rapid test used, and on the type of flu viruses circulating. Also, rapid tests appear to be better at detecting flu in children than in adults. This variation in ability to detect viruses can result in some people who are infected with flu having a negative rapid test result. 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 doctor may test you for flu, not everyone who goes to the doctor with flu-like symptoms will be tested. After evaluating you, your doctor may choose to diagnose you with flu without the need for testing based on your symptoms and his or her own clinical judgement.

COVID-19 and flu co-infection

It is possible to have flu as well as other respiratory illnesses including COVID-19 at the same time. Health experts are still studying how common this can be.

Detecting both flu and COVID-19

There is a test that will check for seasonal flu type A and B viruses and SARS-CoV-2, the virus that causes COVID-19. This test is being used by U.S. public health laboratories for surveillance purposes. Testing for these viruses at the same time will give public health officials important information about how flu and COVID-19 are spreading and what prevention steps should be taken. The test will also help public health laboratories save time and testing materials, and possibly to return test results faster.

What To Do If You Get Sick


CDC

WHAT TO KNOW

  • Most people with flu have mild illness and do not need medical care or antiviral drugs.
  • If you get sick with flu symptoms, in most cases, you should stay home and avoid contact with other people except to get medical care.
  • If you have emergency warning signs of flu sickness, you should go to the emergency room.

Essential Steps to Take When You Get Sick

If you have flu symptoms and are in a group at higher risk of serious flu-related complications, or if you are very sick or worried about your illness, contact your health care provider.

Certain people are at higher risk of serious flu-related complications (including young children, people 65 and older, pregnant women, and people with certain medical conditions). If you are in a group at higher risk and develop flu symptoms, contact your doctor early in your illness. Remind them about your higher-risk status for flu. CDC recommends that people at higher risk for serious flu-related complications should get antiviral treatment as early as possible, because benefit is greatest if treatment is started within 2 days after illness onset.

Deciding if an Emergency Room Visit Is Necessary when Mildly Ill

The emergency room should be used for people who are very sick. You should not go to the emergency room if you are only mildly ill.

If you have emergency warning signs of flu sickness, you should go to the emergency room. If you get sick with flu symptoms and are at higher risk of serious flu complications or you are concerned about your illness, call your health care provider for advice.

Medicines for Treating Flu

There are drugs your doctor may prescribe for treating flu called “antivirals.” These drugs can make you better faster and may also prevent serious complications.

Recommended Time to Stay Home When You’re Ill

You can go back to your normal activities when, for at least 24 hours, both are true:

  • Your symptoms are getting better overall, and
  • You have not had a fever (and are not using fever-reducing medication).

After these two criteria are met, there are some additional precautions that can be taken to protect others from respiratory illness.

Notes

  • People with flu are most contagious during the first 3 days of their illness. Some people, including young children and people with weakened immune systems may be contagious for longer periods of time.
  • Not everyone with flu will have a fever. People with suspected or confirmed flu, who do not have a fever, should stay home from work at least 5 days after the onset of symptoms.

Avoid giving aspirin to children and teenagers

CDC recommends that children and teenagers (anyone aged 18 years and younger) who have flu or are suspected to have flu should not be given Aspirin (acetylsalicylic acid) or any salicylate containing products (e.g. Pepto Bismol); this can cause a rare, very serious complication called Reye’s syndrome.

Critical Actions to Take When You’re Sick

Stay away from others as much as possible to keep from infecting them. If you must leave home, for example to get medical care, cover coughs and sneezes with a tissue. Wearing a mask is an additional prevention strategy that you can choose to further protect yourself and others. When worn by a person with an infection, masks reduce the spread of the virus to others. Masks can also protect wearers from breathing in infectious particles from people around them. Wash your hands often can also help you keep from spreading flu to others. Take other everyday preventive actions to stop the spread of germs. This also can include taking steps for cleaner air and hygiene practices like cleaning frequently touched surfaces.
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