As coronavirus testing ramps up across the state, questions have been raised about the accuracy of some test methods.
Texas Rangers outfielder Joey Gallo received two negative and two positive COVID-19 tests in the span of 10 days. Professional U.S. golfer Cameron Champ received a positive test earlier this month — then tested negative three times in a row only five days later.
Some tests in Texas faced scrutiny last month after one lab turned up an abnormal number of positive tests. And a rapid test used by President Donald Trump has recently come into question.
Here’s what you need to know from health experts about the accuracy of different COVID-19 testing methods.
What are the different types of COVID-19 tests?
There are two main types of COVID-19 tests that can check for an active infection: PCR tests and antigen tests.
PCR, or polymerase chain reaction, tests detect a virus’s genetic material. The most common type of test, it is usually taken with a nasal swab but can also be taken with a throat swab or saliva sample.
Samples are usually sent to a lab to be analyzed, and results take a few days. PCR tests are also commonly referred to as diagnostic tests or molecular tests.
Antigen tests, on the other hand, work by detecting a virus’s proteins. Antigen tests deliver results in a few hours and are taken with a swab from the nose or throat.
Which type of test is more accurate?
Health experts say PCR tests are generally more reliable than antigen tests, which should mostly be used to confirm an active infection.
A PCR test is sensitive and can detect even trace amounts of the virus in a person’s body. However, experts warn that it can take a few days from the time someone is infected for the virus to be present in a person’s throat and nose. So if someone gets a swab too early, the test might miss an infection.
Health experts told the website Healthline that to get the most accurate result, patients should wait eight days after a suspected exposure.
Because PCR tests are so sensitive and can detect the virus for longer periods of time, the test method may return false positives.
“You can’t tell if the person ... [had an infection] three days ago or five months ago,” Dr. Christina Wojewoda, a pathologist at the University of Vermont and vice chair of the College of American Pathologists’ microbiology committee, told Healthline.
But health experts say a false positive is better than getting a false negative, which is more common with antigen tests.
According to the Food and Drug Administration, antigen tests can’t definitively rule out an active infection. Health experts say that generally a positive test result from an antigen test can be trusted, while a negative result would preferably be backed up with a PCR test.
“PCR testing is certainly going to be the most accurate in terms of being able to pick up the disease with a very small number of viral particles,” said Dr. Beth Kassanoff, an internal medicine physician with North Texas Preferred Health Partners and the 2021 president-elect of the Dallas County Medical Society. “The antigen test probably would turn out to be a good test in terms of just sort of broad population screening, because they can probably be done cheaper. … But here it’s not widely available.”
Dr. Eric Cioe-Pena, an emergency medicine physician at Northwell Health in New York City, told the Today show that under ideal conditions, a PCR test is about 99% accurate. Antigen tests, meanwhile, were estimated to be about 93% accurate by Dr. Jake Deutsch, clinical director and co-founder of Cure Urgent Care in New York City.
But ultimately, health experts say that the accuracy of either test depends on several conditions, including how well it’s collected.
Saliva tests, for example, would not be as accurate if a patient had something to eat or drink before the test, Kassanoff said.
And a nasal swab has “to go far enough back that unfortunately it’s uncomfortable for the patient,” she said. “If you’ve got somebody who’s performing the procedure who’s skittish, or just scared of hurting somebody and is not swabbing aggressively enough, then the test is just not going to be valid.”
