LONDON -- The World Anti-Doping Agency defended its test for human growth hormone on Wednesday and accused the NFL players union of being "extremist" for questioning its validity.

WADA director general David Howman said the union was acting "the way they've operated the last few years" in trying to block the introduction of HGH testing in the National Football League.

"I would expect the players association to take a stance which is extremist," Howman told The Associated Press in a telephone interview. "What we've got to do is get to reality and not to a position that is an extremist position."

The NFLPA questioned the HGH test again after Tuesday's ruling by the Court of Arbitration for Sport in the case of Estonian cross-country skier Andrus Veerpalu.

The court lifted the three-year suspension imposed by the International Ski Federation on the two-time Olympic champion for use of HGH, citing "procedural flaws" in the limits established by WADA to determine a positive test.

But the three-person CAS panel also said it believed Veerpalu did take HGH and it backed the WADA testing method as a whole.

"What we have to do is actually look at the decision in a very calculated, objective fashion," Howman said. "What CAS has decided is that the test is OK and what they want is for there to be a bigger population-based study in terms of the impact of it. We'll take that on board and we'll go further."

The NFL players said the decision highlighted its long-running concerns about HGH testing in pro football.

"For almost two years, the NFL players have fought the NFL and certain members of Congress who have publicly referred to the players' insistence on scientific validity and fairness as 'stalling' and 'posturing,'" the NFLPA said in a statement Tuesday. "Today's decision validates the players' demand for scientific validity, full due process rights, and a transparent system."

The NFL responded in a statement that it has been almost two years since an agreement was reached with the players on testing but HGH controls still have not started.

"Surprisingly, the union uses this particular decision to justify and extend into overtime its game of duck and delay," the NFL said.

The NFL and the union agreed in principle to HGH testing when a new 10-year labor agreement was finalized in August 2011. But protocols must be approved by both sides and the players have questioned the science in the testing procedures, delaying implementation.

"There is no contest as to the validity of the test," Howman said. "The test is OK. We have to make that quite strong. There's some extra work to be done. In fact, we've been doing it and we've been saying we've been doing it for the past four or five years.

"It's not a surprise to us. It's just part of what you do in the evolution of things. If there are more blood tests taken, there is more information available to us."

In a prolonged legal battle with the ski federation and WADA, Veerpalu's lawyers tried to align him with the NFLPA in contesting the HGH test.

However, CAS rejected such a link as "irrelevant to the question of the test's validity and reliability" - and suggested the union was motivated by the labor negotiations.

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