Coronavirus tests: which one should you take?
New coronavirus tests are being developed every day. The Trump administration just ordered 150 million rapid antigen tests from Abbott Laboratories, but how do they stack up against other tests like the Polymerase chain reaction (PCR) test? Top infectious disease doctors from Harvard and Johns Hopkins break down the differences between the two tests to determine which diagnostic tool might be better at curbing transmission rates.
With the revelation that President Trump and the First Lady have tested positive for coronavirus, many who may have been in contact with them are being tested using now widely-available rapid tests to see if they too are infected. But how accurate are these quick turnaround tests?
"Tests that are rapid, usually providing results within 30 minutes, are not as sensitive as the traditional test that is sent into the laboratory and generally takes half a day to several days to get a result on," Dr. Romney Humphries, Ph.D., Director of Clinical Microbiology Laboratory at Vanderbilt University Medical Center, told Fox News in August.
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“There is a 75-80% detection rate for rapid testing compared to 90-95% with traditional, lab-based tests," Humphries, also a member of the Infectious Diseases Society of America, added.
Humphries added that the rapid test’s ability to correctly identify those with the disease — test sensitivity — may not always be clinically relevant.
"Some studies have shown that rapid tests miss low levels of virus. A patient may have low levels of the virus if they are early in the course of infection before the virus has had a chance to replicate itself, or at the end of infection when the infection is dying out.”
Humphries, also a professor of Pathology, Microbiology, and Immunology at Vanderbilt University Medical Center, noted that this could be a potential problem with spreading the virus.
"There is a 75-80% detection rate for rapid testing compared to 90-95% with traditional, lab-based tests," Humphries said. (iStock)
“In the former case, missing a positive is problematic, because that is a patient that can then go on an infect others, thinking they are negative. In the latter case, the low-levels of virus may in fact be dead virus, and that patient may not be infectious."
Although the wait time for the results of the two tests differ, the technique appears similar in terms of swabbing the nasal passageway.
According to the U.S. Food and Drug Administration (FDA), the antigen test — commonly known as the “rapid diagnostic test” where results are given within minutes to an hour — detects specific proteins on the virus’s surface. It is usually highly accurate, though more likely to miss an active infection compared to the molecular test, the FDA stated on its website.
Meanwhile, the molecular test, also known as the RT-PCR test, nucleic acid amplification test (NAAT), or diagnostic test, detects the virus’s genetic material and typically can take a day up to a week, depending on the demand on the lab. According to the FDA, this test is highly accurate and usually does not need to be repeated. The FDA advises if you do have symptoms but your rapid antigen test is negative, your health care provider may want to order a molecular test to see if you do indeed have an active infection.
As for which test Humphries prefers, “I think the laboratory-based test with longer results is the preferable approach, when possible. That being said, there are times when this is not possible, and the rapid tests in the right context work well," she said.
“The rapid tests play an important role and can be very valuable in some cases. For instance, a patient who needs trauma surgery can’t wait a day or two for test results because the physicians need to know the results before performing a life-saving, emergency procedure. Many hospitals are using rapid tests for this purpose.”
“Another example is screening people before an event — or possibly using it before going to school — theoretically a rapid test would be valuable in those contexts, although this application of rapid tests is not common," Humphries added.
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The microbiologist cautioned that no test is perfect and each has its limitations.
“One factor that affects every COVID test is getting a really good sample. To collect a proper nasal pharyngeal sample the tester needs to get back enough in the nose to cause a gag reflex because that is where the coronavirus is. They need to go in deep. Basically, the chances for a more accurate test result increase with a better sample," she noted.