The players' union has told the NFL to hold off collecting blood for HGH testing, and the league isn't happy about it. "We informed the NFL [Tuesday] that absent a collective agreement on several critical issues, blood collection is not ready to be implemented on Monday," the NFL Players Association said in a statement released Wednesday. "We have advised the players." The league was preparing to draw the blood samples beginning Monday, although full testing for HGH was not yet scheduled. "We are disappointed in the union's response," NFL spokesman Greg Aiello said. "It is contrary to the terms of the CBA and the agreements reached last Friday with the chairman and ranking member of the House Government Reform Committee. "We are ready to begin educating players on the testing program and collecting samples. This approach was put forward by Congressmen Issa and Cummings," he added, referring to California Rep. Darrell Issa and Maryland Rep. Elijah Cummings, "It is well-reasoned and balances the need to ensure immediate deterrence with the union's desire for further review and education. We know of no reason why these initial steps should not begin next week, and none has been identified by the union." The NFL and the players agreed to begin blood testing for HGH as part of their new collective bargaining agreement reached in late July — but only if the union agreed to the methods. The union has delayed implementing the test, asking for more scientific data to prove it is reliable. One of the key items the NFLPA is seeking is a population study of the test — the data from the athletes who were used to originally set thresholds as to what constitutes a positive test. The union wants to compare that data to a population study on football players; the union believes players could have naturally higher HGH levels above those of other athletes. "I am all for it, as long as it's a test that can be regulated and proved to be very accurate," Packers quarterback and Super Bowl MVP Aaron Rodgers said. "So until we get some information and data that we're very confident in, I don't know that there's going to be anything in the immediate future. I think it'd be great to even the playing field for the rest of the league." Earlier this month, nearly two dozen scientists and lab directors from around the world signed a letter sent to the NFL and the union stating the current test for HGH is safe, scientifically reliable and appropriate for use in professional sports leagues. The letter, obtained by The Associated Press, was dated Oct. 3 and sent to NFL Commissioner Roger Goodell and union Executive Director DeMaurice Smith.