The NFL and the NFL Players Association have tweaked their coronavirus testing procedures in an attempt to separate "false-positive" results from active infections, the league said in a memo sent to clubs Friday.

The policy will apply to any Tier 1 or Tier 2 employee -- including all players and coaches -- who produces a positive test but is asymptomatic and has no known history of a coronavirus infection. If the person meets those criteria, the league will require two additional tests within 24 hours. If both of those tests come back negative, the person will be allowed to resume normal activity.

The change comes less than a week after Detroit Lions quarterback Matthew Stafford was placed on the reserve/COVID-19 list because of one positive test during the league's training camp intake process. Stafford later tested negative three times and was reinstated to the active roster.

Dr. Allen Sills, the NFL's chief medical officer, said Friday that the league has conducted 75,000 tests in the past two weeks and remains committed to evolving the league's policy whenever necessary.

"We've always wanted to make sure that we provide the most accurate test results," Sills said. "In doing 75,000 tests over the first two weeks, you collect a lot of learning. We are constantly looking at all aspects of the protocol and how we can make it better. ...

"What we're trying to do here is be really, radically transparent. We're trying to tell you, 'This is what we're learning and seeing in real time, here are the steps we're making to adjust.' And I expect that process to continue over the course of this season."

The NFLPA announced Thursday on its website that 56 players have tested positive since the start of training camp. Sills declined to confirm that number, however. He said that IQVIA, the league's health data analyst, is in the process of reconciling positive test numbers to understand how many revealed actual infections and how many were either "persistent" -- a result of a previous infection that is no longer active -- or otherwise inconclusive. The league is not using the term "false positive," an NFL adviser said, because the tests are considered accurate but sometimes identify remnants of an inactive infection or, in some cases, an immunological response to a virus other than COVID-19.

To this point, NFL players had been placed on the reserve/COVID-19 list if they had any positive test or if they were determined to have had close contact with someone who was infected.

In the memo, the NFL also reminded teams that all Tier 1 and Tier 2 employees must wear masks inside their team buildings, and that a face shield is not an acceptable substitute. Also, the league continued to encourage players to test out custom Oakley mouth shields that can be attached to their helmets. Another suggested option is to wear a neck gaiter. Neither is required, however.