What are COVID-19 ‘rapid tests’ and how accurate are they? Answers to common questions.
As with everything related to the novel coronavirus, what we know about testing for COVID-19 is constantly changing.
When we hear experts talk about testing and the different types of tests used to slow the spread of the coronavirus, what we’re usually hearing about is molecular diagnostic testing. But then we also hear about testing for antibodies.
And more recently, researchers are pushing widespread antigen testing, known as “rapid testing,” as a key to safely reopening schools and businesses.
A recent NPR report quotes Dr. Ashish Jha, dean of the Brown School of Public Health, as saying that rapid antigen testing will give us the capacity to “actually play offense — go and hunt for the disease before it spreads to identify asymptomatic people before they spread it to others. It really becomes about preventing outbreaks — not just capturing them after they’ve occurred.”
The NPR report lays out a plan of “daily testing targets” — teachers, healthcare workers, nursing home residents and staff, prison inmates, college students and others — for this proactive testing theory. The analysis suggests that the U.S. would need 4.4 million tests every day, and that North Carolina would need nearly 160,000 tests each day to stop outbreaks.
But what exactly is the deal with these rapid antigen tests? How are they different? Are they accurate?
For this Q&A, we relied primarily on research and reporting from the Food and Drug Administration and from the Harvard School of Medicine.
What is a rapid COVID-19 test?
Rapid tests are point-of-care diagnostic tests that use a mucus sample from the nose or throat, but can be analyzed at a doctor’s office or clinic instead of being sent to a laboratory. The results can be available very quickly, sometimes in minutes. In fact, many companies are starting to announce rapid tests that can one day be used at home.
Rapid tests are almost always antigen tests. Traditional diagnostic tests are molecular tests.
What is the difference between an antigen test and a molecular test?
An antigen test detects specific proteins on the surface of the virus. These tests are quicker and less expensive, but have a higher chance of missing an active infection, according to the FDA.
A molecular (PCR or polymerase chain reaction) test detects the virus’s genetic material. These tests require a more complex technology to get results, and it usually takes a day or two to get those results (depending on lab capacity, results may take up to a week).
If an antigen test shows a negative result and you have reason to believe you may have the virus (because of symptoms or exposure), your doctor may order a molecular test to confirm the results.
Which tests are more accurate?
No test is 100% accurate, but the molecular tests are considered to be more accurate than the antigen tests, according to available research.
According to the FDA, an antigen test cannot “definitively rule out active coronavirus infection,” but positive results are “highly accurate” (negative results may require a confirmation test).
Harvard Medical School wrote in August that the reported rate of false negatives with molecular testing is as low as 2% and as high as 37%. A molecular test using a deep nasal swab will have fewer false negative results than samples from throat swabs or saliva, they say.
For antigen testing, Harvard noted that the reported rate of false negative results can be as high as 50%, but that the FDA has granted emergency use authorization for a more accurate antigen test.
In general, some of the issues that may affect the accuracy of a test include ineffective swabbing, contamination or mishandling of the sample, or problems with the testing chemicals, according to the FDA.
If antigen tests are less accurate, why would we use them?
Experts say the value in the rapid antigen tests is in the frequency of the testing rather than the accuracy, with repeated testing recommended. According to Science, getting a false negative in an antigen test two or three times in a row is rare. This strategy lines up with the NPR report mentioned above, which would employ daily testing of those in high-risk areas.
How and where are we currently using rapid tests?
Rapid antigen tests may be used right now some in doctor’s offices and hospitals, but the most visible deployment of the repeated rapid test strategy is by college and professional football teams, who test players and coaches daily to determine if they are healthy for practice and play.
The federal government announced this week it will be sending out 150 million rapid tests to various groups, but distribution to nursing homes so far has been problematic.
Can I buy a rapid test to use at home?
Right now you can’t buy a rapid test to test yourself at home, but experts are sure that is coming. The current tests must be performed by a health professional.
But you can purchase a swabbing kit to take a sample at home, and then mail it to a lab. As with other molecular testing, you will have to wait at least a few days for the results.
What is an antibody test?
Antibody tests were all the rage this spring, but their usefulness is debatable.
Antibodies help your body fight infection. An antibody test uses a blood sample to look for antibodies made by our immune system in response to a virus. It does not look for the virus itself, and it takes several weeks for your immune system to make enough antibodies to show up on a test.
If tested during a certain period of time (within several weeks of infection), the test may tell you if you have had the virus recently, but these tests should not be used to diagnose an active coronavirus infection.
And, as researchers point out, it is currently unknown if the presence of antibodies — which do not appear to remain in the system beyond a few months — means that you are immune to the virus in the future.
This story was originally published October 1, 2020 at 2:56 PM with the headline "What are COVID-19 ‘rapid tests’ and how accurate are they? Answers to common questions.."
