FAQ: Testing for COVID-19

What is the difference between a diagnostic test and an antibody test?

diagnostic test, sometimes called a “viral test,” looks for signs that you have a current infection. An antibody test, or “serology test,” can show that you were previously infected with the virus or vaccinated against the virus.

June 1, 2021

How do we measure test accuracy?

When we measure the accuracy of a laboratory test, we’re actually measuring two different types of accuracy:

  • The test’s sensitivity is its ability to correctly indicate when an individual is currently infected (for a diagnostic case) or has has antibodies to the virus (for an antibody test).
  • The test’s specificity is its ability to correctly indicate when an individual is not infected (for a diagnostic test) or doesn’t have antibodies (for an antibody test).

A test that is very sensitive is less likely to give false-negative results, and a test that is highly specific is less likely to give false positives.

May 11, 2020

What is a “false negative?”

A false negative is a test result that is wrong, because it indicates the person is not infected when they really are, or that they don’t have antibodies when they actually do.

May 11, 2020

What is a “false positive?”

A false positive is a test result that is wrong, because it indicates the person is infected when they really are not or that they have antibodies when they really don’t.

May 11, 2020

How does the diagnostic test work?

The test most commonly used to diagnose COVID-19 is a molecular test, or PCR test. It works by detecting genetic material from SARS-CoV-2, the virus that causes COVID-19. When someone is infected, they have this genetic material in their nose and upper throat. The test uses a sample that is collected with a swab from an area of the nasal passage where viral particles are likely to be present.

The Food and Drug Administration (FDA) has recently authorized a second type of diagnostic test known as an antigen test. Rather than looking for genetic material from the virus, the antigen test looks for molecules on the surface of the virus. Antigen tests are relatively inexpensive and can be run in about 15 minutes without specialized equipment. 

May 13, 2020

Who should get tested?

If you have symptoms of COVID-19, you may need to be tested. Contact your own primary care provider for advice on what to do next.

March 2, 2022

How accurate is the diagnostic test for COVID-19?

The PCR test for COVID-19 works by detecting genetic material from SARS-CoV-2, the virus that causes COVID-19. The genetic material from SARS-CoV-2 cannot be confused with the genetic material from other viruses, so the COVID-19 diagnostic test is highly specific. This means it almost never gives a false positive. If you are tested for COVID-19, and the test comes back positive, you can be very sure that you are infected with this virus. The new antigen test for COVID-19 is also very specific and rarely gives a false positive.

Unfortunately, neither test is equally sensitive. If the specimen collection is not done perfectly, or if you are in an early stage of infection or already partially recovered, your nasal-swab sample might not contain enough viral material to come back positive. There are many stories about patients who tested negative soon after their symptoms began, only to test positive on a test done later. It is clear that the PCR test is more accurate at detecting early-stage infections, and there are early indications that the antigen test may be better at identifying patients who are already recovering. 

However, because false negative results on diagnostic tests happen relatively often, a negative result should not give you a sense of false security. If you have any symptoms of COVID-19, it is safest to assume you are infected and act accordingly, even if your diagnostic test comes back negative.

May 13, 2020

How does an antibody test work?

An antibody test is a blood test that looks for immune molecules, or antibodies, specifically targeted to fighting SARS-CoV-2, the virus that causes the COVID-19 illness. An antibody test for COVID-19, if accurate, could indicate if you had previously been infected with the virus, even if you never had any symptoms.

May 11, 2020

Do antibodies provide immunity to reinfection?

This is something we still don’t know about this virus, as the Centers for Disease Control and Prevention (CDC) recently reiterated. With other viruses, immunity after infection can range from lifelong and complete (measles) to nonexistent (HIV). Antibodies to other coronaviruses — the ones that cause the common cold as well as SARS and MERS — persist and are protective against reinfection for several years. This has led many scientists to hypothesize that that antibodies to SARS-CoV-2 may be able to provide protection for at least some period of time. 

But antibody tests aren’t like pregnancy tests — a simple “yes” or “no.” Instead, they are “titer tests” that indicate specific levels of antibodies in an individual’s blood. Unfortunately, we don’t know what level of antibodies might be required for immunity from reinfection. We also don’t know if people with antibodies can still spread the virus to others, even if they are immune themselves.

There is proof that some people who have recovered from COVID-19 have been reinfected. People who have had COVID-19 should still be vaccinated.

January 11, 2022

How accurate is the antibody test?

The Food and Drug Administration (FDA) has been allowing companies to market COVID-19 antibody tests without requiring them to submit validation data proving that these tests gave accurate results. As a result, of the more than 100 COVID-19 antibody tests currently available, only a handful have been FDA-approved.

But the accuracy of even these tests depends on the percentage of people in the population who have actually been exposed to the virus. For example, in a population where the prevalence of infection is 5%, a test with 95% specificity and 95% sensitivity will return the same number of false positives as true positives, making any individual result no more useful than the flip of a coin. 

For a complete explanation, watch MIT Professor Michael J. Cima demonstrate the math that explains how a test’s sensitivity and specificity may combine with low population prevalence to make an individual antibody test relatively useless.

May 11, 2020