NFL Commissioner Roger Goodell said Monday he remains hopeful that the players’ union will agree to blood-testing for human growth hormone at some point this season.

“We’re ready to go,” Goodell said. “I think the science is proven. It’s a valid test and we’re hoping that we’ll get the green light from the union.”

The league and union agreed. as part of their new collective bargaining agreement, that players would be tested annually for HGH and would be subject to additional random testing throughout the year. The two sides targeted the start of the regular season last month to begin the testing.

But the league and union first must agree to the testing methods. Negotiations on the details have stalled, with union officials saying they have not received the information they need about the reliability and safety of the test.

“We have very robust testing on the non-HGH side of performance-enhancing testing,” Goodell said. “. . . Now this is slightly different obviously because it involves drawing blood. But we’ve been focused on this for well over a year with the Players Association.”

The union did not immediately respond to a request for comment.

Several members of Congress have urged the two sides in recent weeks to begin the testing, and a House committee has invited representatives of the league and union to attend a meeting to discuss the issue. Goodell said he expects the meeting to occur within the next two weeks.

“It’s written in the CBA that we’re committed to do it by the regular season,” Goodell said. “So it’s in there. You can read it.”

A new blood test for HGH, which reportedly can detect the substance up to three weeks after its use, has been developed but has not received final validation by the World Anti-Doping Agency. The current blood test for HGH can detect the substance up to three days after its use.

Goodell spoke to reporters in downtown Washington after addressing a conference of neurosurgeons. Goodell told the conference that the NFL has made great progress in its efforts to reduce the rate and severity of concussions suffered by players, but work remains to be done.

“I think we’ve made tremendous progress,” he said later. “It comes in a variety of different ways. Overall it’s about changing the culture, not just in the NFL but in sports in general. . . There’s tremendous awareness of the injury and the seriousness of the injury and the fact that if it occurs, it has to be treated by medical professionals. I think that’s a huge step, because that wasn’t the case even a decade or two ago.”

Goodell said he believes “players are playing the game differently” because of rules aimed at eliminating hits to the head of an opponent in a defenseless position. He defended the rule changes, saying they have made the game safer while the sport’s popularity continues to grow.

The NFL’s leaders will need a full cycle of offseason practices and training camps, Goodell said, before they evaluate the effectiveness of the new rules in the sport’s labor agreement that limit contact on the practice field. Goodell said the league also wants to take a full season to assess this year’s new safety-related kickoff rules.

The sport’s medical and competition committees continue to study long-term issues such as the possible elimination of the three-point stance, Goodell said. “They just haven’t felt that it was a significant move that was going to really change the game from a health and safety standpoint right now,” he added.

Goodell told the conference that the vast majority of the increased spending on medical research in the labor deal will be devoted to brain injuries.