What To Know About Flu Tests—When You Need One, and What To Do if You Test Positive

If you begin to feel run down and have a sore throat, runny nose, and a fever during the fall and winter months, then you might naturally assume you have the flu. But, how do you tell for sure you have the flu and not another illness that has similar symptoms to the flu? Is there a definitive test you can take that will tell you if what you're feeling is the flu?

How Common Is the Flu?

Between 2010 and 2020, the Centers for Disease Control and Prevention (CDC) said that the flu resulted in 9 million to 41 million flu illnesses each year—and even that is just an estimate.

The truth is, the CDC doesn't know exactly how many people get the flu annually, for a variety of reasons—mainly because many people don't seek medical care for flu symptoms, and even when they do seek treatment, healthcare providers may not test for the flu because it doesn't change how a person is treated.

That, and other factors like people who seek care when their flu can no longer be detected through a flu test, contribute to the fact that even the data we have may underrepresent the true burden of flu in the US on any given year.

A flu test is the only real way to know whether or not you or a loved one experiencing flu-like symptoms actually has the flu—and it can be a powerful tool, when reported, to help experts understand just how bad any given flu season is affecting the population.

But getting tested for the flu can be kind of intimidating—no one really wants to get a swab up their nose, after all. Here's what you need to know about flu tests, including who typically needs one, how long it takes to get results back, and what to do if you test positive for influenza.

How Do You Know if You Need a Flu Test?

Your need for a flu test will come down to the symptoms you present to your healthcare provider, and how they decide to diagnose those symptoms. The most common signs and symptoms of the flu include:

• Fever and chills (note: not everyone with flu will have a fever)
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headache
• Fatigue
• Vomiting and diarrhea (this is usually more common in children)

You may notice that the symptoms listed above are also very common to those seen with COVID-19. If if your healthcare provider decides to test for COVID-19 first, and it comes back negative, they may move on to a flu test, said Robert L. Murphy, MD, a professor of infectious diseases at Northwestern University Feinberg School of Medicine.

Most people don't actually need a flu test, but some of the reasons a healthcare provider might do a flu test are:

• The person has risk factors for flu complications
• There's a local flu outbreak that the health department is monitoring
• The infected person has close contact with others who have a high risk of complications from the flu

Are There Different Types of Flu Tests?

Yes, there are several types of flu tests and each requires either a nasal swab—where a provider swipes the inside of your nose—or a throat swab, said Cassandra Pierre, MD, MPH, an infectious disease physician at Boston Medical Center.

Even more specifically, some tests go further back in the nose, while the rapid test only goes right inside, said Anjali Mahoney, MD, MPH, a family medicine specialist with Keck Medicine of USC.

Rapid Tests

There are several different ways to test for influenza and some tests are available at a healthcare provider's office or pharmacy. The most common test for the flu is called a "rapid influenza diagnostic test" (RIDT). These RIDTs detect the part of the virus that stimulates an immune response, known as an antigen.

Another type of flu test, called a "rapid molecular assay," detects the actual genetic material of the flu virus. Both of these tests are termed "rapid" because they can provide results in 10 to 15 minutes (RIDTs) or 15 to 20 minutes (rapid molecular assays).

Tests in Hospitals or Public Laboratories

Even more accurate than RIDTs and rapid molecular assays are the tests most often found in hospitals or public health laboratories (those places have more specialized labs than your average urgent care practice or healthcare provider's office). Those tests are known as reverse transcription polymerase chain reaction (RT-PCR) tests, viral cultures, or immunofluorescence assays. These results also take a little longer—up to several hours.

While no test is 100% accurate, Dr. Pierre said flu tests have a high degree of sensitivity and specificity, meaning they're likely to show a positive result if you're infected and a negative result if you're not infected. In some cases, Dr. Pierre said, providers will give a rapid test first in hopes of treating the flu as soon as possible, then send a sample for RT-PCR testing to the lab to be sure of the diagnosis. Either way, your provider's goal is to help you feel better and prevent the spread in the community. "The reason flu tests are important is because there's treatment," said Dr. Murphy.

Tests for COVID-19 and Flu

Also important: There is a test that can check for both seasonal flu and COVID-19—these tests were sent to public health laboratories in August 2020, and the CDC continues to make and distribute the kits throughout the US. (Note: You can also have the flu and COVID-19 at the same time, making these dual tests even more important.)

So What Should You Do After a Positive Flu Test?

If your flu test is positive, your healthcare provider will provide treatment options and the laboratory will likely report the results to the state Department of Health because the CDC collects information on flu infections year-round in the U.S.

Get Treatment

Your healthcare provider will also decide whether or not you need treatment. One treatment for influenza is the antiviral drug Tamiflu—also called oseltamivir—which can reduce the duration of symptoms and prevent serious side effects, but Dr. Mahoney said it's only effective within the first 48 to 72 hours of symptoms.

Other antiviral medications, such as Zanamivir, Peramivir, and Baloxavir can also be prescribed to treat the flu. The key is to talk with a healthcare provider as soon as you notice symptoms so that if you need treatment it can be provided during the time when it's most effective.

While anyone can take Tamiflu or other antiviral medications, healthcare providers often prescribe it for people who:

• Are hospitalized
• Are very sick but not hospitalized
• Have a higher risk of severe illness or hospitalization from influenza (such as pregnant people and adults over age 65)

"We may even treat those patients if they don't test positive for flu," explained Dr. Mahoney.

In general, Dr. Pierre said, patients tolerate Tamiflu well, but some people experience nausea, vomiting, diarrhea, and headaches as side effects.

Prevent the Spread

In addition to any antiviral drugs you're given—or if you're given the OK to treat yourself at home with rest and fluids—you should still take precautions to make sure you don't make anyone else sick. The CDC recommends:

• Limiting your contact with others
• Washing your hands frequently
• Keeping commonly-touched surfaces clean
• Covering your nose or mouth when you cough or sneeze

You should also do your best to stay home (except if you need medical care)—a good rule is to wait at least 24 hours until your fever is gone to go out in public.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.