3 Types of COVID-19 Tests

Since COVID-19 began its rampant spread in 2020, we have learned that testing is vital to track and contain the spread of the disease. Many options are available for testing, including at-home tests and those conducted at doctors' offices and healthcare facilities. Some tests are based on mucus samples, while others use saliva or blood. Given the various options, how do you know which test is right for you? And do they all work the same way? Or tell you the same information?

We will start with the basics. As of August 2022, there are two categories of COVID-19 tests:

• Diagnostic tests that look for active infection with SARS-CoV-2, the virus that causes COVID-19, in your mucus or saliva
• Blood tests that show past infection through the presence of antibodies—evidence that your immune system has encountered the infection before.

Here's what you should know about these COVID-19 tests, how they're used, and what they can tell you.

Getting the Right COVID-19 Test

The right test depends on the goal. Goals can be to confirm an active COVID-19 infection, identify individuals who are asymptomatic (don't show any symptoms) or pre-symptomatic to follow the Centers for Disease Control and Prevention (CDC) isolation guidelines, or determine whether someone previously had COVID-19.

"There is not a lone testing approach that is going to meet every need and solve every problem," said Daniel D. Rhoads, MD, section head of microbiology at the Cleveland Clinic.

If your goal is to figure out if you currently have the virus, which is most often the case, you'll need a diagnostic test. Diagnostic viral tests are usually either rapid tests or laboratory tests. Rapid tests produce results in minutes and can include antigen tests but also some nucleic acid amplification tests (NAATs), which identify RNA sequences (the genetic material of the virus), as the CDC explains.

However, if you want to find out whether you've had an infection in the past, you'll want to take an antibody test. Antibodies are proteins the immune system makes in response to disease. Sometimes these tests are also called serology tests.

Getting the Test Results Right

The accuracy of a test is determined based on two different things: sensitivity and specificity.

Sensitivity is the test's ability to correctly detect the disease. With a test that is highly sensitive, there is a low chance you will get a false-negative result. In other words, if you have the disease, there is a very high chance that you will test positive for the disease. High sensitivity ensures that cases of disease are not missed, which is essential in stopping the spread of disease.

Specificity works the other way. When a test is highly specific, it will have the ability to correctly identify people who do not have the disease. If a test is highly specific, the rate of false positives will be low. This means if you do not have the disease, there is a very high chance that you will get a negative result from a highly specific test.

Diagnostic Tests

Let's say your healthcare provider orders a diagnostic test to confirm or rule out you are infected with SARS-CoV-2. Even among those tests, important differences exist. According Dr. Rhoads, there are a couple of ways to detect the virus. Some tests look for a piece of the coating of the virus—they're called antigen tests—and other tests called molecular tests detect nucleic acid (genetic material such as RNA) belonging to SARS-CoV-2.

Laboratory Tests

Laboratory tests are RNA tests that use a variety of methods to detect the virus. One type of NAAT is a polymerase chain reaction (PCR) test, a lab technique used to detect the virus's genetic material, explains the U.S. Food and Drug Administration (FDA). Laboratory tests are highly sensitive but can take days to get a result.

According to the CDC, SARS-Cov-2 RNA can still be detected in people who have recovered from COVID-19. This means that the RNA tests can remain positive for up to three months. However, the CDC says that these patients are not likely infectious.

"And neither antigen nor RNA testing predicts when someone is no longer contagious," Dr. Rhoads said. Since the virus can be detected after a person recovers from COVID-19, the CDC recommends relying on symptoms rather than test results to determine when someone can safely end any quarantine measures.

If you are getting a laboratory test, in most cases, a healthcare provider will collect mucus from your nose or throat using a specialized swab. (Some molecular tests use saliva, which people may find more comfortable.) Turnaround time varies from minutes to days or longer, depending on whether the sample is analyzed at the testing site or sent to an outside lab. Generally, laboratory-based NAATs have higher sensitivity than rapid or at-home tests, according to the CDC.

Also, at-home test kits are available, allowing people to collect their own sample (mucus or spit) and get results within a few minutes, explains the CDC. These test kits are easy to use and perhaps less intimidating than long-swab testing used in healthcare settings. Plus, one study from July 2020 published in the Journal of the American Medical Association suggests that when people collect their own sample, self-testing yields results that can be just as accurate as those obtained by healthcare workers.

Antigen Tests

Antigen tests are rapid tests that consistently have a quicker turnaround time than RNA tests. Also, antigen tests are cheaper to produce, making them ideal for screening large numbers of people, like at airports, a 2020 article in the journal Nature points out.

But, antigen tests are often less accurate than RNA tests. That means antigen testing is more prone to false-negative results and more likely to miss cases of active infection.

From a patient's point of view, antigen testing works in much the same way as molecular testing. Your healthcare provider will swab the back of your nose or throat to collect a sample for testing. If you test positive, it's probably correct: Antigen tests are highly accurate. The problem is, these tests are more likely to miss active infection. If you have COVID-19 symptoms but test negative, your healthcare provider may order a molecular test just to rule out a false negative.

Like some NAATs, antigen tests can be performed right at your healthcare provider's office, meaning you don't have to go to a lab for testing. Sindhu Aderson, MD, of Chicago-based Northwestern Immediate Care, said, "these point-of-care tests are mostly used in emergency departments, doctor's offices, and outpatient clinics."

Antibody Test (Serology Test or Blood Test)

This test looks for antibodies to SARS-CoV-2. Antibodies are proteins your immune system produces to fight off a foreign invader, such as a virus. A COVID-19 antibody test cannot diagnose active SARS-CoV-2 infection, but it can determine if you've had the virus. Antibodies also can be detected if someone has been vaccinated for COVID-19. So, while the test may indicate a past infection, it can also show an immune response from being vaccinated, as the CDC explains.

Again, antibody testing isn't recommended to diagnose a current infection. This is because it can take one to three weeks after the infection before your body makes antibodies, according to the CDC. If you test too early—while your immune system is still mounting its defense—the test may not provide an accurate result.

Sometimes antibody testing is done along with viral testing when someone seeks care late in the course of their illness. It may also help confirm a diagnosis of multisystem inflammatory syndrome in children, a condition linked to COVID-19.

There are no FDA-authorized, at-home antibody tests. You'll have to see a healthcare professional, who will take a blood sample via a finger prick or a blood draw from a vein in your arm. The vast majority of these tests are performed at a central lab, which can take a couple of days to process.

But in September 2020, the FDA approved the first antibody point-of-care test, making it possible for doctor's offices, hospitals, urgent care centers, and emergency rooms to get an answer within 15 minutes using blood from a person's fingertip.

If you have antibodies to the virus that causes COVID-19, does that mean you have immunity against future COVID-19 infections? The CDC emphasizes that antibody testing cannot determine immunity from COVID-19 after vaccination.

Antibody testing has a role to play in the pandemic because it can provide information on the prevalence of the infection in the population and the frequency of asymptomatic infection, per the FDA.

Getting Tested

In most cases, you will want to know if you have a current infection with SARS-CoV-2, so you'll need a diagnostic test. Either you can have a healthcare provider test you or you can take a diagnostic test at home.

As of August 2022, the FDA has approved both molecular and antigen COVID-19 diagnostic tests that can be used at home. Most at-home tests are antigen tests, which are not as good at detecting the virus as molecular tests. You are more likely to get a false negative with an at-home antigen test. So if you're testing at home with an antigen test, the FDA recommends that you repeat the test at least once 48 hours after your first negative test result.

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.