There are now more than 10 million people in the U.S. who have tested positive for the coronavirus. And as the number of cases soars across the country, the demand for testing is increasing.
Dr. Daniel Rhoads, section head of microbiology at the Cleveland Clinic in Ohio, explains what types of diagnostic tests are available and how reliable they are.
The PCR Test
The PCR test — short for polymerase chain reaction — is a molecular test that looks for viral RNA, which is the coronavirus’ genome, Rhoads says. The PCR test is the most common type of molecular test, he says, making it “the gold standard” because of how sensitive it is at detecting COVID-19’s genetic material.
Most PCR tests use a nasal or nasopharyngeal swab to collect a mucus specimen sample. When a PCR test comes back positive, it’s “very accurate,” he says, however there is still a small subset of patients who have the virus and test negative.
The Food and Drug Administration says molecular tests are “typically highly accurate.”
The Antigen Test
The antigen test, also known as the rapid test, also comes in the form of a nasal swab, Rhoads says. But for a rapid test, the swab needs to go deeper in the back of the nose than the PCR test in order to pick up as much of the virus as possible.
For quick results, the whole specimen is used in an antigen test, he says. Results can come back in a matter of minutes.
“If you want to be as sensitive as possible, then you want to do a PCR test,” he says. “If you have somebody with symptoms and you're pretty sure they have COVID and you want a really quick result, then an antigen test is a reasonable approach.”
The FDA says positive results from antigen tests are “usually highly accurate,” however antigen testing is “more likely to miss an active COVID-19 infection compared to molecular tests.”
Spit tests use the PCR platform for COVID-19 testing, he says. But it can be harder to find this option of testing because of supply chain issues and constraints of the laboratory, he says.
While spit tests are molecular tests, which Rhoads calls “the gold standard,” the accuracy of the spit test depends on how the saliva is collected, he says.
“But there is the potential that saliva testing could be as good as a nasal swab if it's collected the right way and tested the right way,” he says.
Antibody tests — a blood test — can determine whether a patient has been infected with the coronavirus at some point, Rhoads says. In many cases, a patient is no longer showing symptoms of the disease, so antibody tests shouldn’t be used to diagnose a patient with active COVID-19, the FDA says.
Antibody testing is important clinically and in vaccine studies in order to see one’s antibody response, he says. It’s also helpful for studies looking into how many people have become sick with the coronavirus but never received a test. For epidemiological and surveillance purposes, antibody tests can reveal interesting patterns and information about the public, he says.
At this time, the FDA says researchers are still testing whether the presence of antibodies equals COVID-19 immunity.
In terms of testing across the country, the U.S. is better off now than when the pandemic struck in March, he says. But supplies and laboratory systems are still not at the speed and capacity they need to be to handle the volume.
“We're in a much better place than we were,” he says, “but I don't think anybody would say we're where we need to be yet.”
This segment aired on November 10, 2020.