How long does a flu test take—and can you trust the results?

Your head hurts. Your muscles ache. You want to climb into bed and pull the covers over your head. 

When this unwelcome visitor known as influenza makes its appearance—as it does for 3 million people each year—it’s important to know what to do (and when). Early treatment with antiviral medication makes it less likely you’ll experience complications, such as hospitalization. Before treating you for the flu, though, your healthcare provider may use a rapid influenza test to determine whether your illness is caused by influenza. Many other viral illnesses can cause flu-like symptoms and the antiviral medications for influenza do not work on other infections.

A flu test is quick and easy: It takes only a minute in the doctor’s office to collect a sample and 15 to 30 minutes to get a result. While the test doesn’t detect every case of the flu, it is reliable when it tells you that you have the virus. 

What is the flu? 

Influenza is an acute respiratory illness caused by influenza A or B viruses that routinely spreads through the U.S. during the winter months. Frequent symptoms include fever, headache, fatigue, muscle aches, sore throat, cough, congestion, chills, and a runny or stuffy nose. While this respiratory infection can be mild for many, it is serious and even life-threatening for some. Those most at risk for severe complications are young children, older adults, pregnant women, those with chronic health conditions, or those with weakened immune systems. Many people diagnose themselves with the flu and manage the illness at home with over-the-counter pain relievers. However, getting a flu test and a prescription antiviral medication can reduce the severity and length of your symptoms.

Is there a test for the flu?

Yes! You should get a flu test as soon as possible, preferably within four days after first experiencing symptoms. It’s important to get a flu test because detection of influenza virus can reduce unnecessary laboratory testing for other infections and stop inappropriate antibiotic use. It will also encourage the effectiveness of infection prevention and control measures and increase the appropriate use of antiviral medications.

Several tests can detect the influenza virus, but only two are recommended by the Infectious Diseases Society of America (IDSA) for use in non-hospitalized people:

• Rapid molecular assay, also called a nucleic acid amplification test
• Rapid influenza diagnostic test (RIDT), also called an antigen detection test

While the RIDTs are more common, rapid molecular assays are preferred by experts over RIDTs because they are more accurate. 

You’ll see other types of tests mentioned, such as real-time reverse-transcription polymerase chain reaction (RT-PCR), viral cultures, or serologic testing, but these are best used in hospitalized patients when a quick result is not required. 

Since flu-like symptoms are similar to those of COVID-19, your physician may test for both during the ongoing pandemic. It’s possible to have both the flu and coronavirus infection at the same time. Some lab testing sites can test one sample for both the influenza and SARS-CoV-2 viruses, but when getting a test in the doctor’s office, you’ll likely need to give two samples for separate flu and COVID-19 tests. 

How long does the flu test take?

For both types of rapid flu test, your healthcare provider may use a nasal swab to wipe the inside of your nose or throat swab on the back of your throat. This testing portion takes only a few seconds, then the swab will be tested and you’ll wait for your results. 

The time to get your results varies a bit depending on which test your provider uses. The rapid influenza diagnostic test (remember, this antigen test is less accurate) gives the fastest result in 10 to 15 minutes. A rapid molecular assay will give you results in 15 to 30 minutes. You’ll simply wait in the doctor’s office, and your healthcare provider will return once results are available. 

Clinicians could choose instead to squirt some saline in your nose and collect the liquid (known as nasal aspirate) for diagnostic testing instead of using the swab.

How accurate is the rapid flu test?

Rapid influenza diagnostic tests accurately detect the presence of the flu virus antigen between  50% to 70% of the time and are better at detecting influenza A than influenza B. Some modifications to the test can increase accuracy to about 80%; this increased accuracy is now required by the Food and Drug Administration (FDA).

In practical terms, this means if 10 people with the flu were tested, the test would miss two to five of those and falsely report that they did not have the virus. This is known as a false negative result. RIDTs are not the preferred method of testing due to the high likelihood of these false negative results. 

The RIDTs, if positive for influenza, are pretty accurate—meaning if the test is positive, the chance you have influenza is great. Roughly 1 out of 10 tests will result in a false positive, which for a test done in the office in 15 minutes is very good. A false positive is less likely to occur during flu season (when lots of people have the flu) and more likely to occur outside of flu season when fewer people have the flu.

*Data provided by the CDC

Rapid molecular assay tests tend to be more accurate than RIDTs, with a 90 to 95% likelihood that the test will detect the virus in those who have the flu. Using the example above, if you have 10 people who have the flu, the RIDT may miss anywhere from two to five of those diagnoses, but the molecular assay would miss only one, if any. 

Expert tips for getting a flu test

Here are a few things to note when seeking care for your flu symptoms: 

1. Get a flu test and a COVID-19 test as soon as you are symptomatic. Getting a flu test earlier in the course of illness can increase the likelihood of an accurate result.
2. If you do have the flu, antiviral medications may lessen the length of symptoms and the severity of your illness. These medications are more effective if started within two days of your first symptoms.
3. If you have a negative test, discuss with your healthcare provider whether you still may have the flu even though the test results didn’t report it. 
4. If you get a positive flu result, discuss with your provider whether an antiviral medication, such as Tamiflu (oseltamivir), would be helpful.  
5. If you are diagnosed with the flu, be aware that it is very contagious. Take precautions. Prevent the spread. Wash your hands frequently. Wear a mask. Distance. Work from home. Take other public health measures.
6. Keep in mind—the best way to prevent the flu is to get your annual flu shot. 
7. Remember, antibiotics do not treat viral infections, such as influenza A virus, influenza B virus, or the SARS-CoV-2 virus. 

Finding a flu test near you

Rapid influenza tests are available at most urgent care or walk-in clinics as well as your primary care provider’s office. If you’re experiencing flu symptoms, call ahead and ask whether flu tests are offered at that location. Your primary care office should be your first call. Describe your symptoms and ask if they can see you that day.

Insurance may cover the cost of the test. If you don’t have insurance, your healthcare provider may charge anywhere from $50 to $100 in addition to the cost of the office visit. 

Remember, the best way to prevent seasonal influenza is to get a yearly flu vaccine. While the flu shot can’t guarantee that you won’t get sick, it dramatically reduces your chances, according to the Centers for Disease Control and Prevention (CDC). If you do get the flu after having the vaccine, you’ll likely have a milder course and are less likely to experience serious problems.