COVID-19 Resources

Reasons for a False Positive or False Negative COVID-19 Test Result

Aug. 17, 2020

UC Health’s chief of Pathology and Laboratory Medicine says that the chance of a false COVID-19 test result depends on the type of test.

The type of COVID-19 test that UC Health uses — a PCR test, which looks for genetic material of the novel coronavirus — is the gold standard test.

False positives and false negatives, therefore, aren’t common, said Dani Zander, MD, chief of Pathology and Laboratory Medicine at UC Health.

“A false-negative test can happen if one has a very low viral load,” Dr. Zander said.

A viral load represents the amount of the virus in any given testing sample, like on a nasopharyngeal swab.

Someone may have a very low viral load in the first couple of days after they’ve become infected. Their viral load may also be low at the end of the course of their infection.

Other tests, such as a type of rapid test called an antigen test, are “less sensitive” to detecting the virus, Dr. Zander said.

False positives are also uncommon in PCR tests. It can happen in a situation where there’s cross-contamination, she said.

“Laboratories work very hard to minimize the risk of cross-contamination, so it’s not common,” Dr. Zander said.

Conversely, false positives are much more common with antibody tests.

“That’s been one of the limitations [of antibody testing],” she said. “If you live in a low prevalence area like we do, a lot of the positive antibody test results are false-positive results.” There are a few reasons for this, such as the test picking up antibodies for other types of coronaviruses.