COVID-19 diagnostic testing

September, 25th, 2024

, Overview, ,

COVID-19 diagnostic testing shows current infection with the virus that causes coronavirus disease 2019 (COVID-19). The U.S. Food and Drug Administration (FDA) approved the following types of tests for diagnosing COVID-19.

Molecular tests

These tests look for genetic material from the COVID-19 virus.

Polymerase chain reaction tests, shortened to PCR tests, are molecular tests. The lab technique used is called reverse transcription polymerase chain reaction, shortened to RT-PCR. You also may see this type of test called an NAAT test, short for nucleic acid amplification test.

PCR tests are more accurate than the other type of COVID-19 test, called an antigen test. PCR tests may be done at home. But they are more likely to be done by a healthcare professional and processed in a lab.

Antigen tests

These tests look for viral proteins called antigens.

Antigen tests also may be called rapid COVID-19 tests or at-home COVID-19 tests. These tests give a quick result.

Antigen tests are accurate, but they are less accurate than PCR tests. This is especially true if you don’t have symptoms. If you take an antigen test and are negative for COVID-19, take another antigen test after 48 hours to check the results.

Tests for more than one virus

An RT-PCR test called the Flu SC2 Multiplex Assay can show any of three viruses at the same time: the COVID-19 virus, influenza A and influenza B. Only a single sample is needed to check for all three viruses.

This test has the same risks of false results as do other COVID-19 tests.

Why it’s done

In the U.S., you might need a COVID-19 diagnostic test if:

• You have COVID-19 symptoms, such as fever, cough, tiredness or shortness of breath. If you have symptoms, test right away.
• You were exposed to the COVID-19 virus. Wait at least five days after you’ve been exposed to take a test.
• A healthcare professional or your public health department suggests a test.

Testing before you plan to be with someone who has a high risk of serious illness from COVID-19 helps prevent the spread of the COVID-19 virus.

If you’ve had COVID-19 in the last 30 days and don’t have symptoms, you might not need to test even if tests are suggested.

Some people who have COVID-19 don’t have symptoms, called asymptomatic. But they can still spread the virus to others. People with no symptoms can be tested.

If people with no symptoms have a positive test result, they should follow guidelines for staying away from others. This helps keep the virus from spreading.

Risks

A COVID-19 diagnostic test can have a false-negative result. This means that the test didn’t show you have the virus even though you do. You risk spreading the virus to others if you don’t take care, such as wearing a face mask, when you’re near other people.

The FDA requires that rapid antigen diagnostic tests catch at least 80% of infections. To lower the risk of false-negative results, the FDA recommends testing more than once.

Rarely, COVID-19 rapid antigen tests can give false-positive results.

False-positive results mean the test results show that you have the virus when you don’t. A false-positive is rare and thought to happen in less than 1% of results.

In some cases, a false-positive result could be caused by a manufacturing error. Early evidence suggests that this result may be more common in people with a history of autoimmune disease.

Don’t use at-home COVID-19 tests that the FDA has not cleared for use. They may give the wrong results.

How you prepare

In the United States, at-home COVID-19 tests are available. You can buy tests in stores, pharmacies or online. It’s useful to have tests before you need them. And it’s good to have more than one test in case you need to repeat the test.

The FDA approves COVID-19 tests. On the FDA website, you can find a list of approved tests and when they expire. COVID-19 tests also are available from healthcare professionals, some pharmacies and clinics, or at community testing sites.

When taking a test at home, read the directions that come with the test carefully. Follow the instructions exactly to get as accurate a result as possible.

If you go somewhere to get tested, wear a face mask. Wear a mask even if you don’t have symptoms.

What you can expect

COVID-19 tests use a sample of mucus from your nose or throat or a sample of saliva. You might collect the sample yourself for an at-home test. Or a healthcare professional might collect the sample.

• Nose or throat swab. Most often, tests use a long nasal swab to get the sample. You or a healthcare professional puts the swab into one nostril at a time and turns it. A healthcare professional might swab the back of your throat to collect the sample.
• Saliva sample. Some locations test using spit, called saliva. A saliva sample may be a bit less sensitive than a mucus sample that’s taken using a long nasal swab. But a saliva test is easier to do. You spit into a tube several times to provide a sample of your saliva to test. The tube then goes to a lab.

A healthcare provider also may test for other illnesses, such as the flu, that have similar symptoms and that you may have.

Results

Your COVID-19 diagnostic test result could be positive or negative.

Positive result

This means that you have an infection with the virus that causes COVID-19. Positive test results most often are correct, even if you have no symptoms. You can spread the virus to others even if you have no symptoms.

You’ll need to stay away from other people, including others in your home who aren’t sick. You can go back to your daily activities when your symptoms have been getting better for at least 24 hours and it’s been 24 hours since you’ve had a fever without taking medicines that lower fevers.

You might still be able to spread the virus even though you feel better. To prevent spreading the virus, wear a mask for five more days. Keep your distance from others. Take another home COVID-19 test if you plan to be around others indoors.

If you get a fever or start to feel worse again, start over with staying home and staying away from others. Repeat the cycle of instructions.

If you have severe symptoms of COVID-19 or a health condition that lowers your ability to fight disease, your healthcare professional may tell you to stay away from other people longer than five days.

Negative result

This means that you likely don’t have an infection with the COVID-19 virus. But you may have a false-negative test result. If you have symptoms or were around someone who had COVID-19, stay away from others.

Even if you don’t have symptoms, test again after 48 hours. If it’s still negative, and you think you have COVID, you can test a third time after another 48 hours. Or you can get a molecular test or call your healthcare professional.