Which test is best for COVID-19?

Now that we're well into the COVID-19 pandemic, the steps we need to take to effectively control the outbreak have become clear: conscientious prevention measures like wearing masks, washing hands, and distancing; widespread testing with quick turnaround times; and contact tracing for people who test positive to help stop the spread. Combined, these are our best bets while awaiting better treatments and widespread vaccination.

So, which tests to use?

Start here: There are tests that diagnose current infection and tests that show whether you previously had SARS-CoV-2, the virus that causes COVID-19.

The FDA has granted emergency use authorization (EUA) for hundreds of COVID-19 diagnostic tests. This allows makers to market tests without receiving formal FDA approval because there is a public health emergency.

Because this novel coronavirus is indeed novel, and COVID-19 is a new disease, information about available tests is incomplete and testing options keep changing. Tests vary in terms of accuracy, cost, recommended use for people of various ages, and convenience. This blog post reviews features of the major types of tests. For more detailed information on individual tests, check the FDA's updated list of authorized tests, which has links to the test makers' websites.

Diagnostic tests for current infection

If you want to know whether or not you have the virus, there are two types of tests: molecular tests and antigen testing.

Molecular tests (also called PCR tests, viral RNA tests, nucleic acid tests)

How is it done? Nasal swabs, throat swabs, and tests of saliva or other bodily fluids.

Where can you get this test? At a hospital, in a medical office, in your car, or even at home. The FDA has authorized a home molecular test that requires no prescription.

What does the test look for? Molecular tests look for genetic material that comes only from the virus.

How long does it take to get results? It depends on lab capacity. Results may be ready within hours, but often take at least a day or two. Much longer turnaround times are reported in many places.

What about accuracy? The rate of false negatives — a test that says you don't have the virus when you actually do have the virus — varies depending on how long infection has been present: in one study, the false-negative rate was 20% when testing was performed five days after symptoms began, but much higher (up to 100%) earlier in infection.

The false positive rate — that is, how often the test says you have the virus when you actually do not — should be close to zero. Most false-positive results are thought to be due to lab contamination or other problems with how the lab has performed the test, not limitations of the test itself.

A molecular test using a nasal swab is usually the best option, because it will have fewer false negative results than other diagnostic tests or samples from throat swabs or saliva. People who are in the hospital, though, may have other types of samples taken.

Antigen tests

How is it done? A nasal or throat swab.

Where can you get these tests? At a hospital, a doctor's office or at home (the FDA has authorized a home antigen test that requires no prescription).

What does the test look for? This test identifies protein fragments (antigens) from the virus.

How long does it take to get results? The technology involved is similar to a pregnancy test or a rapid strep test, with results available in minutes.

What about accuracy? False negative results tend to occur more often with antigen tests than with molecular tests. This is why antigen tests are not favored by the FDA as a single test for active infection. Because antigen testing is quicker, less expensive, and requires less complex technology to perform than molecular testing, some experts recommend repeated antigen testing as a reasonable strategy. As with the molecular test, the false positive rate of antigen testing should be close to zero.

Tests for past infection

An antibody test can show if you have previously been infected with the COVID-19 virus.

Antibody tests (also called serologic testing)

How is it done? A sample of blood is taken.

Where can you get these tests? At a doctor's office, blood testing lab, or hospital.

What does the test look for? These blood tests identify antibodies that the body's immune system has produced in response to the infection. While a serologic test cannot tell you if you have an infection now, it can accurately identify past infection.

How long does it take to get results? Results are usually available within a few days.

What about accuracy? Having an antibody test too early can lead to false negative results. That's because it takes a week or two after infection for your immune system to produce antibodies. The reported rate of false negatives is 20%. However, the range of false negatives is from 0% to 30%, depending on the study and when in the course of infection the test is performed.

Research suggests antibody levels may wane over just a few months. And while a positive antibody test proves you've been exposed to the virus, it's not yet known whether such results indicate a lack of contagiousness or long-lasting, protective immunity.

The true accuracy of tests for COVID-19 is uncertain

Unfortunately, it's not clear exactly how accurate any of these tests are. There are several reasons for this:

• We don't have precise measures of accuracy for these tests — just some commonly quoted figures for false negatives or false positives, such as those reported above. False negative tests provide false reassurance, and could lead to delayed treatment and relaxed restrictions despite being contagious. False positives, which are much less likely, can cause unwarranted anxiety and require people to quarantine unnecessarily.
• How carefully a specimen is collected and stored may affect accuracy.
• Because these tests are available by EUA, the usual rigorous testing and vetting has not yet happened, and accuracy results have not been widely published.
• A large and growing number of laboratories and companies offer these tests, so accuracy may vary.
• All of these tests are new because the virus is new. Without a long track record, assessments of accuracy can only be approximate.
• We don't have a definitive "gold standard" test with which to compare them.

How much does testing cost?

Cost varies widely, from under $10 to hundreds of dollars, depending on whether your health insurance will cover the test, or if you'll be paying out of pocket. It also depends on which test you get and where it's done.

It's important to ask about cost ahead of time, especially if you are told to go to the ER or a private testing facility. Some communities offer free tests. Go online or call your town or city government to find out about this.

The bottom line

If we had fast, highly accurate, and inexpensive home testing, we could repeatedly test ourselves before going to work, heading off to school, or otherwise spending time near others. If the tests were more accurate, we could more confidently rely on testing to know when it's safe to relax distancing, masking, and other protective measures. But we aren't there yet.

While we're lucky to have reasonably accurate tests available so early in the course of a newly identified virus, we need better tests and easy access to them. All tests should undergo rigorous vetting by the FDA as soon as possible. Ideally, they should be covered by health insurance or offered free of charge to those who are uninsured. Lastly, widely available tests and short turnaround times for results are essential for effective contact tracing and getting this virus under control while awaiting widespread vaccination.

Follow me on Twitter @RobShmerling

For more information about coronavirus and COVID-19, see the Harvard Health Publishing Coronavirus Resource Center.