AUSTIN, Texas - Can a negative COVID-19 test result provide certainty that someone will not pass the virus on to others? The answer may be more complicated than you think.
"Any test is better than no test," said Dr. Anas Daghastani, CEO of Austin Regional Clinic and an internal medicine physician.
Still, there is one test that's more accurate than the others.
"So, we're lucky to have both tests and, if they're positive, they're both accurate. It's when they're negative, when they come back negative, where the PCR is a few percent more accurate than the antigen," Daghastani said.
It all has to do with what the tests are looking for and how long it takes those markers to show up in an infected person. "PCR is looking for the DNA of the virus, antigen is looking for the protein of the virus," said Daghastani.
Antigen test results can be returned in as little as 15 to 30 minutes, which is why they are so popular. Molecular, also called PCR tests, usually take at least 24 hours because they have to be run by a lab.
According to the Food and Drug Administration, "antigen tests are more likely to miss an active COVID-19 infection compared to molecular tests." That's especially true if those tests are taken on the first couple of days of infection.
"If the first test is positive, that's PCR or antigen, I think we can feel almost 99.99 confident that that's a positive test, it is a COVID. But, if the first test is negative, and your symptoms are suggestive of COVID, then probably it's not a bad idea to confirm," Daghastani said.
Delaying tests for a few days after possible exposure is also recommended. A study published in August by the medical journal Annals of Internal Medicine found that in the first four days of infection, prior to the onset of any symptoms, the probability of a false negative result of a PCR-type test was between 67 to 100 percent. On the day symptoms began the likelihood of a false negative was closer to 38 percent. By day three of symptoms, the false-negative rate reached its lowest point at 20 percent. Not perfect, but when combined with other precautions, doctors said it's still helpful.
"You use a mask, it reduces your risk a little bit. You try and stay six feet apart, reduce your risk a little bit more. You try to do it outdoors, you reduce risk a little bit more. And then you take a test, you reduce a little bit more. It's really the cumulative impact of all of those pieces," said Daghastani.
Antibody tests are another kind of test that can be used, but only to check if someone was previously exposed to the virus. Those tests shouldn't be taken until a few weeks after possible infection according to medical professionals and, even then, doctors aren't sure how long antibodies stay in a person's system.