Ex-Marlins first baseman Mike Jacobs tested positive for HGH earlier this year while playing for the Rockies’ Class AAA team. (Doug Benc/GETTY IMAGES)

Anti-doping experts and political figures have taken great pains in the last two weeks to laud Major League Baseball for a new agreement to test players for human growth hormone beginning next spring. When that happens, MLB could beat the NFL to the distinction of becoming the first U.S. pro sports major league to implement HGH analysis, after years of entrenched resistance to blood testing.

But baseball’s program contains a loophole that could allow use of HGH from opening day until the end of the World Series, experts say, at a time when a rigorous testing program could make a big impact.

For the first year at least, MLB will conduct more than 1,000 HGH tests — more than any Olympic sports body ran last year — but only in spring training and then again in the offseason. There will be no testing during the 162-game regular season and playoffs, unless league officials can establish “reasonable cause” to target a player, such as obtaining evidence he placed an order for the substance.

“Why should they be getting a free pass during the season? . . . You’d have an enormous opportunity to dope with growth hormone and never be detected,” said Gary Wadler, the immediate past chairman of the World Anti-Doping Agency’s prohibited-list committee. “I applaud them for taking a step in the right direction. I would applaud them with gusto if they took the step they should be taking: year-round [HGH] testing.”

The loophole — along with the continued stalemate between the NFL and its players on HGH testing, and no indication from the NBA that its new collective bargaining agreement will include HGH tests — suggests that implementing full-fledged HGH testing programs in U.S. pro sports leagues may be nearly as difficult as getting them to agree to the testing itself.

Which is unfortunate, officials say, because the test has gained credibility in recent months, after years of criticism.

A positive turn for testing

HGH, which is illegal without a prescription in the United States, is believed to have a number of benefits for athletes, including promoting healing and muscle growth and enhancing the effectiveness of steroids. A number of prominent MLB players were linked to HGH use in a report authored by former Sen. George Mitchell in 2007, which intensified calls for pro sports leagues to begin HGH testing.

But MLB and NFL players resisted for years, raising concerns about drawing their blood while noting that the available HGH test seemed laughably weak. Indeed, in their first five years of use worldwide, from 2004 through 2009, HGH tests did not catch a single athlete. The lack of positives proved embarrassing to anti-doping officials, who insisted the test would catch users eventually but had no evidence to back that claim.

Suddenly, they do.

The first hit came in February, 2010, when a British rugby player tested positive. That was followed by a surprising spate of positives in the 12-month period from September 2010 through August 2011, when six international athletes — a skier, a triathlete, a Canadian college football player, a shot-putter, a cyclist and minor league baseball player Mike Jacobs — tested positive for HGH.

At least three accused athletes — including Jacobs — admitted using the drug. (In many cases, athletes contest the results of positive tests and seek resolution through arbitration.) The run of positives — and the confessions — helped reduce resistance to the test from those who questioned its reliability and effectiveness, according to Rob Manfred, MLB’s executive vice president of labor relations.

“It was an ongoing educational effort in terms of getting people comfortable with this test,” Manfred said. “When you have positives and you have athletes that admit [using] it, that is a form of practical validation.”

But Manfred said players balked at in-season testing out of concern that giving blood on game days would detract from their performance. Wadler called that argument “laughable,” saying the amount of blood drawn — a standard doctor’s office blood sample — is too small to affect performance.

Manfred said MLB could commence in-season testing as soon as 2013 if players agreed to it during an offseason review of the program required by the new CBA.

“Hopefully, we can reach an agreement that results in in-season testing. . . . I don’t think we’re done on this topic,” he said.

Lacking implementation

The NBA meantime, discussed the issue with its players during collective bargaining talks, according to a league spokesman, but there has been no public indication HGH testing is a priority for either side. The NBA has escaped close scrutiny on this issue for the simple reason that none of its players has been named in any of the drug scandals of recent years that have focused often-intense criticism on MLB and the NFL.

The NFL has not put its HGH blood-testing program into effect even though the league and the players’ union agreed to testing when they completed their 10-year labor agreement in August.

The two sides agreed to both annual and random testing. Each player would be tested annually in training camp and would be subject to random testing year-round, including during the regular season. The two sides had hoped to start testing when the season opened.

But the labor deal also stipulated that the league and union first had to agree to the details of the testing program, and those deliberations have stalled. The players have demanded a population study of NFL athletes to determine growth hormone norms for them. They claim norms could be higher for them than for other athletes because of their size and might trigger false positives under the current testing regimen.

WADA Science Director Olivier Rabin said the agency has observed no changes in natural growth hormone values in relation to the size of athletes. He pointed out that players large and small, from sports such as rugby and Australian football to table tennis, have been tested.

The NFL declined to comment this week through a spokesman. But NFL Commissioner Roger Goodell and other league officials have said repeatedly that they believe the testing should be taking place. The union, which has faced congressional pressure on the issue, did not respond to requests for comment on the status of the deliberations.

“Baseball’s taken the lead,” said Tom Davis, the former Virginia Republican chairman of the House Government Reform Committee who held landmark hearings on drugs in baseball in 2005. “I hope football will follow.”

A ‘significant’ step

WADA Director General David Howman said MLB’s program, while flawed because of the lack of in-season tests, would immediately make it among the world leaders in HGH testing on a numerical basis. Many Olympic sport governing bodies, he said, have failed to implement adequate HGH testing; the agency has been pressing them to make blood samples constitute at least 10 percent of their total anti-doping samples.

MLB’s plan “is significant,” Howman said. “Many Olympic sports throughout the world have not taken an agressive stance on HGH, and we’ve been very critical of that.”

Howman said the world track (IAAF) and cycling (ICU) organizations were among the leaders in blood testing and HGH testing in particular — and neither ran 1,000 HGH tests in 2010. The ICU, which takes thousands of blood samples from its athletes, ran 483 HGH-specific tests and the IAAF conducted 552. There were 3,425 HGH tests administered worldwide in 2010.

Rabin attributed the rise in positives to the increase in unannounced, out-of-competition HGH testing. The test itself is largely the same as the one introduced at the 2004 Summer Games in Athens.

Critics have long said a flaw in the test is its short detection window of 48 to 72 hours, but WADA said it soon hopes to validate a test for HGH that will expand that window to two to three weeks.

“We are at the stage,” Rabin said, “when we are absolutely confident about the test.”