Exactly one month has passed since the NFL instituted blood testing for human growth hormone -- a test experts say is almost impossible to fail.

An Associated Press analysis of the testing protocol approved by the league and the NFL Players Association after more than three years of wrangling found that only the most reckless or uninformed player would seem to have a chance of getting caught using HGH.

Of the 2,798 HGH tests from various sports analyzed last year at labs accredited by the World Anti-Doping Agency, zero turned up positive.

The USADA's Travis Tygart is an outspoken critic of the NFL's new plan. "You pretty much have to be a fool to test positive" at a team facility, he said. David Butler II/USA TODAY Sports

"You pretty much have to be a fool to test positive'' at a team facility, said Travis Tygart, the CEO of the U.S. Anti-Doping Agency.

Tygart and other leaders of the anti-doping movement say testing alone will not find cheaters -- especially because the current test detects synthetic HGH in a person's system for only 48 hours.

The NFL and the NFLPA initially paved the way for HGH tests in their 2011 labor deal, but it wasn't until this year that the parameters were established: Every week, five randomly selected players on eight randomly selected teams -- a computer makes the picks -- are told to give blood. There are 950 tests during the preseason, regular season and playoffs combined, 385 more during the offseason.

WADA, though, says HGH testing should increase during what it calls "out-of-competition periods."

Among the deficiencies in the NFL program, critics say:

• Timing and location of the majority of testing is too predictable.

• The selection of who gets tested is actually too random, rather than targeting players who might be thought more likely to be using HGH.

"The players' union invited [USADA and WADA] to be part of the process," NFLPA spokesman George Atallah said. "When they refused to be transparent, they forfeited their right to be credible critics."

Asked whether the union is satisfied with the first month of testing, Atallah replied: "Given how new things are, there are always things that can be worked out or need to be improved. But generally, it's working like it's supposed to."

According to USADA's Tygart, however, it's far from sufficient.

Positive tests are "just highly unlikely, because they've known it's coming and probably stopped well in advance, to have it clear from their system," Tygart said. "The question then becomes: Are athletes going to take a chance?"

One tenet of a strong anti-doping program is no-notice testing, including the idea that an athlete could be tested anytime and anywhere. Cyclist Lance Armstrong, who denied doping for years before admitting it, complained frequently about the numerous visits sample collectors made to his house at all hours of the day.

But the NFL's in-season HGH testing takes place at team facilities, which means players know they would need to provide a blood sample only on days when they're there. Another limitation: Testing is not allowed on game days. During the offseason, a player is given up to 24 hours to set up a meeting, which could allow HGH to leave his system.

Asked if the union is satisfied with the first month of testing, NFLPA spokesman George Atallah said, "Given how new things are, there are always things that can be worked out or need to be improved. But generally, it's working like it's supposed to." AP Photo/Carolyn Kaster

"There has to be an unpredictability piece," said Larry Bowers, chief science officer for USADA. "If you look at the WADA code approach, the way they get whereabouts information for the athletes, they try to be unpredictable. It's a very important piece of what we do."

One effective part of the NFL program, experts acknowledge, is that players can be tested more than once during a season, which would prevent them from feeling they're "clear" after one test.

NFL spokesman Brian McCarthy noted the rules allow for increased testing if the league finds reason for it, such as a history of violations or a criminal investigation that collects evidence of doping.

"If information is brought to our attention and there is law enforcement activity, then that would be taken into account," McCarthy said.

In 2007, for example, then-Patriots safety Rodney Harrison and then-Cowboys quarterbacks coach Wade Wilson were suspended by the NFL after the Albany (N.Y.) District Attorney's office investigated an Internet drug operation.

Another missing piece in the NFL system, according to USADA, is a lack of state-of-the art "longitudinal" testing, which tracks an athlete's body chemistry over weeks and years.

"We have utilized longitudinal analysis for many years, albeit not in the prospective manner of the more recent 'athlete passport'-type programs," McCarthy said.

In interviews with the AP, players expressed a mix of curiosity, suspicion and nonchalance about the HGH tests.

"I'm definitely someone who doesn't really like needles, but at the same time -- and the PA might not be happy with me saying this -- I'm all for it because guys, I'm sure, could get away with it for so long. I want it to be a level playing field," said Washington Redskins linebacker Ryan Kerrigan, who added he was not one of the players selected last month for his team's first go-round of HGH testing.

"I don't know anyone specifically that's on it," Kerrigan said, "but I'd be ignorant to think there aren't guys on it."