Which test is best for COVID-19?

Robert H. Shmerling, MD

Senior Faculty Editor, Harvard Health Publishing

Now that we’re several months into the COVID-19 pandemic, steps we need to take to effectively control the outbreak have become clear: conscientious prevention measures like handwashing and distancing, widespread testing with quick turnaround times, and contact tracing. None of these is easy to maintain over a prolonged period. But combined, they are our best bets while awaiting better treatments and an effective vaccine.

So, which tests to use?

The many types of tests available are sowing considerable confusion. Unfortunately, because this novel coronavirus is indeed novel, and COVID-19 is a new disease, information about these tests is incomplete and the options for testing keep changing. But here’s what we know now about tests designed to diagnose a current infection, and those that show whether you previously had the virus.

Diagnostic tests for current infection

If you want to know if you are currently infected with the COVID-19 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.

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 the same day, but usually take at least a day or two. Throughout the pandemic, especially lately, delayed turnaround times of up to a week or two have been reported in many places.

What about accuracy?  False negatives — that is, a test that says you don’t have the virus when you actually do have the virus — may occur. The reported rate of false negatives is as low as 2% and as high as 37%. 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 deep 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.

You may have heard about pooled testing, in which multiple samples are combined and a molecular test is performed on them. This could speed up the testing of large numbers of people and reduce the number of tests needed.

If a pooled test is negative, the people whose samples were combined are told they have a negative test and individual testing is unnecessary. But if the pooled sample tests positive, each of the individual samples that were taken will then be tested to see which person(s) is responsible for the positive pooled result.

This approach may be particularly helpful in settings where the number of infections is low and declining, and most test results are expected to be negative. For example, in a community where the infection seems to be under control and reopenings of schools and businesses are planned, pooled testing of employees and students could be an effective strategy.

Antigen tests

How is it done?  A nasal or throat swab.

Where can you get these tests?  At a hospital or doctor’s office (though it is likely home testing will soon be available).

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?  The reported rate of false negative results is as high as 50%, which is why antigen tests are not favored by the FDA as a single test for active infection. However, the FDA recently provided emergency use authorization for a more accurate antigen test. 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. According to one test manufacturer, the false positive rate of antigen testing is near zero. So, the recent experience of Ohio Governor Mike DeWine, who apparently had a false-positive result from an antigen test, is rare.

Tests for past infection

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.
  • None of these tests is officially approved by the FDA. They are available because the FDA has granted their makers emergency use authorization. And that means the usual rigorous testing and vetting has not 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. At the date of this posting, more than 170 molecular tests, two antigen tests, and 37 antibody tests are available.
  • 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.

The bottom line

Unfortunately, getting a test for COVID-19 can be confusing, because the options are rapidly changing and tests from many companies are being marketed. Despite the current limitations of testing, we’re lucky to have reasonably accurate tests available so early in the course of a newly identified virus. Imagine where we’d be if that was not the case.

Still, we need better tests and better access to them. And all tests should undergo rigorous vetting by the FDA as soon as possible. Lastly, widely available tests and short turnaround times for results are essential for effective contact tracing and getting this virus under control.

Follow me on Twitter @RobShmerling

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

Related Information: Harvard Health Online

Comments:

  1. Alex

    Is the definition of false positives and false negatives correct? Usually false positives are defined as the possibility of a positive test result given one does not have the disease, not the possibility of one does not have the virus given a positive test result. (Sorry if this is confusing, I am looking at this from a statistics point of view and am trying to do some basic calculations, but I understand if this article is made to be understood more easily)

    • Robert H. Shmerling, MD
      Robert H. Shmerling, MD

      Hi, Alex – thanks for your question. Yes, the definitions of false-positives and false-negatives are correct but I agree, the language around this can be confusing. For false-positive results, it’s the situation in which someone who doesn’t have the disease but has a positive test result; this can also be stated the other way around: a positive test result for someone who doesn’t have the disease.

      They are not probabilities (or “possibilities” as in your question) – they rely on knowing the test result and diagnosis and are based on sensitivity (positive test results among all who have the disease) and specificity (negative test results among all who don’t have the disease). A highly sensitive test has a low false-negative rate. A highly specific test has a low false-positive rate. As examples: For a test with 90% sensitivity, the false-negative rate is 10%. If the specificity is 98%, the false-positive rate is 2%.

      You may be thinking of predictive value – these are probabilities in which you’re starting with a test result (not a diagnosis) and looking at the liklihood that the diagnosis is present. For example, how likely is it that a person with a positive test has the disease (positive predictive value)? Or, what’s the chance that a person with a negative result does not have the disease (negative predictive value)?

      I hope this helps!

  2. Mike O

    Thank you! This clear and concise assessment of current testing options was very helpful as my family decides how to proceed after a possible exposure.

  3. J. Michael Carter

    Thanks for the COVID-19 testing summary. It’s becoming increasingly frustrating to remain optimistic as I seek an accurate rapid test kit for my family. Reading articles about how every Fisher Island resident in Florida has been tested (along with staff) and watching Dr. Vin Gupta praise an actual test kit (but couldn’t voice the company’s name). The punches to the gut continue in the capitalistic system that takes care of those who have the most 1st, 2nd, and 3rd! I’m married, in my late 50s, parents and in-laws living, 4 kids, and 2 grandkids; I can only hold and love on my wife and youngest child. Like many others, I can afford to purchase whichever rapid test I deem the most suitable for my family’s needs. The age old American SOP of I’m not important enough, or sick enough to qualify me to be in that exclusive circle. As an American living in the world’s largest capitalistic society, I should have access to the same products as those deemed the wealthiest and or the most important. So, is America really as capitalistic as portrayed? What a dangerous, empathy lacking, selfish, and very hypocritical time the majority of Americans find ourselves currently living.

    Thanks for allowing me to vent and thanks again for explaining the various testing methods in laymen terminology.

  4. shrabani sarkar

    Thanks for the information

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