No single test for COVID-19 can provide 100% assurance that you haven't acquired the virus in the last few days.
It's a widely acknowledged fact among doctors and public health officials, including those at the U.S. Food and Drug Administration, who require the makers of all COVID-19 tests to print disclaimers to patients saying they could have the illness even if the test comes back negative.
"There may be substantial transmission before onset and even by someone who never reports symptoms," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
But widespread misunderstanding about the meaning of COVID-19 test results still exists, underscored publicly this week by statements from White House Coronavirus Task Force chairman — Vice President Mike Pence. He said Tuesday he could tour Mayo Clinic without a protective mask because he and his entourage are tested "regularly" for COVID-19.
"I don't have the coronavirus," Pence said Tuesday, though no test yet presented to the FDA can confer that level of complete assurance. By Thursday, Pence was wearing a protective mask when he toured a retrofitted GM plant in Kokomo, Ind.
The gold-standard diagnostic tool used in laboratories today to detect evidence of COVID-19 is molecular RT-PCR testing.
But even molecular testing is not definitive. While positive results from a PCR test are seen as "very likely" being accurate, disclosures say, there's a greater risk of getting a false-negative result.
PCR tests require a lot of things to go right — correct reagents are needed, specimens and chemicals need to be stored correctly, samples need to be properly drawn.
The virus can also be missed in a person who has had it for only a few days. The "limits of detection" and analytical specificity and sensitivity for molecular tests vary widely.
"False-negative results are consequential," researchers at the Mayo Clinic wrote earlier this month. "Individuals with these results may relax physical distancing and other personal measures designed to reduce the transmission of the virus to others."