The league has asked the New Jersey lab BioReference to investigate the results "while the clubs work to confirm or rule out the positive tests." The NFL did not identify the teams or say how many tests were positive.
The Jets canceled a walk-through on Saturday night but had a full practice Sunday morning after the previously positive tests came back negative. The Bears moved their practice scheduled for Sunday morning to the afternoon. The Cleveland Browns called off their workout.
Buffalo Bills general manager Brandon Beane said tests in the Northeast had gone "haywire," and called it "a lab issue and not a true issue with our guys currently" after several Bills were held out of practice.
The NFL uses BioReference for all of its COVID-19 testing, though tests are handled by labs throughout the nation to ensure teams get results quickly - hopefully within 24 hours. Heading into this weekend, there had been four confirmed positive tests for players who were at training camps.
"Clubs are taking immediate precautionary measures as outlined in the NFL-NFLPA's health and safety protocols to include contact tracing, isolation of individuals and temporarily adjusting the schedule, where appropriate," the NFL said in a statement. "The other laboratories used for NFL testing have not had similar results."
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The Bears were alerted Sunday morning about the test findings. "We followed additional NFL-NFLPA testing protocol and confirmed all nine results as false positives," the team said in a statement.
Beane stressed the need for transparency throughout the Bills' organization and said some players were on a conference call with the NFL's chief medical officer, Dr. Allen Sills.
"We had an open dialogue with our players," he explained. "We even had some of our leadership crew of players on with Dr. Sills this morning just hearing directly with him, who's aware of all these tests that have gone a little bit haywire here in the Northeast.
"And I think that was good for them to hear, because naturally, if you're going to be out there tackling and blocking and things like that, passing the ball around, people might be unnerved if we got some guys missing."
The Browns said initial results from the lab indicated multiple "presumptive positive cases" that included coaches, players and staff.
"Although, we have received indication from the NFL that they are investigating irregularities in lab results that have also impacted other clubs, out of an abundance of caution, we will immediately follow our comprehensive Infectious Disease Emergency Response plan, and pause any activity in our building today and conduct meetings virtually," a Browns spokesman said. "As per plan procedure, the team is also diligently reviewing proximity data for contact tracing and instructed self-isolation for those who tested positive and others as appropriate, even though they all remain asymptomatic.
"Our facility is currently undergoing a thorough deep-cleaning and disinfecting process and is closed to all personnel for the day."
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