FDA: Antigen tests for COVID-19 are rapid but can lead to false positives

The U.S. Food and Drug Administration is alerting clinical laboratory staff and health care providers that false positive results can occur with antigen tests for the virus that causes COVID-19.

In a letter to stakeholders, the FDA said Tuesday that while antigen tests can be used for the rapid detection of SARS-CoV-2, false positive results can occur, especially if users don’t follow the instructions.

“The FDA is aware of reports of false positive results associated with antigen tests used in nursing homes and other settings and continues to monitor and evaluate these reports and other available information about device safety and performance,” the letter said.

A Boston-area infectious disease expert said the antigen tests are good for large scale screening, when used properly, but must be followed up with more accurate testing.

“If you are testing a population at low risk, it’s fine to do these tests for screening,” said Dr. Daniel Kuritzkes, chief of the Division of Infectious Diseases at Brigham and Women’s Hospital. “But when you get a positive result, you want to confirm it with a more specific test, like a PCR (polymerase chain reaction) test, using a nasal swab.”

PCR tests are more sensitive and more specific, but also slower and more expensive, Kuritzkes said.

The FDA said laboratories should expect some false positive results to occur, even when very accurate tests are used for screening large populations with a low prevalence of infection.

The agency said health care providers and clinical laboratory staff can help ensure accurate reporting of test results by following the instructions for use of a test and key steps in the testing process as recommended by the Centers for Disease Control and Prevention.

If the test components are not stored properly, for example, this can affect the performance of the test, the FDA said.

Reading the test before or after the specified time also could result in false positive or false negative results. In addition, processing multiple specimens in a batch may make it more challenging to ensure the correct incubation time for each specimen, the FDA said.

The agency also warned against, “the risks of cross-contamination when testing patient specimens, which can cause false positive results.”

from Boston Herald https://ift.tt/2TNHmxG