A Look at the Three Most Common Types of COVID-19 Tests

July 17, 2020

by Ricardo Maldonado, M.D.

During our peak in April, when our community was basically on lockdown, many asked what we needed to do to reopen the country. It was clear then, and now, that our ability to test and identify those with the infection was and is the game changer. Identifying a person with a COVID-19 infection can help with prevention of further transmission.

At the beginning of the pandemic, the U.S. had very limited testing, which was a huge disadvantage, to say the least, in our fight against COVID-19. Our testing abilities have greatly improved nationwide and will continue to improve as the pandemic progresses.

EAMC is performing COVID-19 testing on all patients being admitted, regardless of whether they have symptoms of COVID-19. Since some people may have COVID-19 without symptoms, also called asymptomatic, we want to know the status of every patient that comes into the hospital so our team can safely care for them.

There are several COVID-19 tests available at EAMC as well as other places in our area. Below is a comprehensive review of the three most commonly available tests.

PCR test (the best diagnostic test)

The PCR test is used to diagnose people who are currently sick with COVID-19, and is the one that is most commonly heard about.

This test uses a sample of mucus, typically taken from a person's nose or throat. The test may also work on saliva, a method that is still being studied. The test looks for the genetic material of the coronavirus, which is detectable when a person is actively infected.

These are the most reliable of the tests, however, there is no perfect test. It is possible to have a very low level of the virus in the body even with a negative test result.

The turnaround time for this test depends on whether a test can be done “in house,” which can take minutes to hours, or needs to be sent to a special laboratory outside the testing center or hospital, which could take 24 hours or several days.

Antigen test (the second-best diagnostic test)

The antigen test identifies people who are currently infected with COVID-19. This test is what some refer to as the “rapid test,” and it is a good test to use to screen people.

Antigen tests can identify the virus in nose and throat secretions, which it does by looking for proteins from the virus, as opposed to the diagnostic test, which looks for genetic material. This technology is the same technology used in a doctor’s office for rapid strep testing.

The antigen test should provide results in just a few minutes. As a result, they are typically used to screen people in hospitals, certain workplaces, such as long-term care facilities and nursing homes, or in other instances where it is important to find out quickly whether someone is currently at risk of spreading COVID-19.

The antigen test is not as accurate as a PCR test, and negative antigen test results may need to be followed up with a PCR test for diagnosis.

Antibody test

The antibody test is a blood test that looks for antibodies, which are produced in the body in response to an infectious agent such as a virus. Antibody tests identify people who have previously (IgG) or recently (IgM) been infected with COVID-19. Antibodies generally arise any time from four days to more than a week after a person has had an infection, so they are not used to diagnose current infections.

These tests generally produce results in a few minutes, based on a drop of blood taken from the finger. Some research labs use a more sophisticated antibody test, called an Elisa (Enzyme-linked immunoassay), that are more accurate, but not as widely available.

There are more than 120 antibody tests on the market. The Food and Drug Administration has allowed them to be marketed without FDA authorization or under EUA (Emergency Use Authorization), so quality is a concern.

In general, these tests are not reliable enough for individuals to act based on the results. Antibody tests should not be used to make return-to-work decisions, cannot determine if a patient is still contagious or not, and do not guarantee immunity from future COVID-19 infections.

Generally, healthy people who had a mild illness with COVID-19 will not be contagious in about 10 to 14 days. No test is usually needed to return to normal activities, as long as a minimum of 10 days has passed from positive testing. Also, it is important to know that the best time for testing depends on the incubation period. Testing early after an exposure will not rule out a COVID-19 infection, as most cases will be positive between the fifth and tenth day after exposure. Obviously, the exposed person can be tested once they begin to have symptoms.

Ricardo Maldonado, M.D. is an Infectious Diseases specialist and is the sole practitioner with East Alabama Infectious Disease. He joined the medical staff at EAMC in 2009. Dr. Maldonado is leading the clinical response to COVID-19 at East Alabama Medical Center.