Lamont Peterson, center, defeated Amir Khan in December for the WBA super lightweight and IBF junior welterweight belts. (Nick Wass/Associated Press)

Lamont Peterson’s representatives acknowledged Tuesday morning that the Washington boxer had tested positive for a banned substance, and said they were working with pathologists and medical specialists to confirm the origin of the result in order to absolve the unified champion of impropriety.

News of Peterson’s positive test, which took place in March, cast serious doubt on his scheduled rematch with Amir Khan on May 19. Peterson had been set to put his World Boxing Association super lightweight and International Boxing Federation junior welterweight belts on the line at the Mandalay Bay resort in Las Vegas.

The Nevada Athletic Commission, which has final say on whether the fight will take place as scheduled, is expected to make a ruling on the matter Wednesday or Thursday.

Richard Schaefer, chief executive of Golden Boy Promotions, said in a conference call on Tuesday afternoon that Peterson’s drug test revealed specimens that “are consistent with the administration of an anabolic steroid such as testosterone.” Golden Boy handles much of Khan’s publicity, as well as that for the fight itself.

Peterson had requested Olympic-style drug testing, which includes blood samples, be in place leading up to the rematch with Khan, and his team contracted the Las Vegas-based Voluntary Anti-Doping Association to oversee such procedures. The Nevada State Athletic Commission issues only urine testing.

Jeff Fried, Peterson’s attorney, called the failed drug test an “isolated occurrence” in a statement released to reporters.

“We are singularly focused on compiling the necessary medical supporting documentation for submission to the Commission and VADA and for their determination that the appropriate exemption and extenuating circumstances permit the fight to proceed as scheduled,” Fried said in a telephone interview later Tuesday.

The Associated Press reported late Tuesday night that Fried told Nevada Athletic Commission officials in a letter that the positive test was the result of an “inadvertent” failure to disclose medical treatment last November for low testosterone levels.

Medical professionals who spoke in general terms not related to Peterson’s case cited low testosterone levels as a potentially mitigating factor for a positive test. Peterson has not indicated publicly he has such a disorder.

Barry Hunter, Peterson’s lifelong trainer, said on Tuesday in a telephone interview that his camp was “staying positive, very optimistic,” but did not offer specifics regarding the failed test.

“I can tell you one thing,” Hunter said. “The person that you see, the person you know, the person in front of you — Lamont Peterson — would never do something like that, nor did he.”

Hunter added that Peterson’s camp is “going through this particular process right now, and I think when it’s over, everyone will be happy with what they hear. In fact, I know they will.”

The positive test came approximately four months after Peterson beat Khan at the Walter E. Washington Convention Center on Dec. 10 via a 12-round split decision. Khan and his team vehemently disputed the outcome, which included Khan having two points deducted for pushing, and petitioned both sanctioning bodies to have the result overturned or order an immediate rematch.

The WBA mandated a rematch, but shortly before the IBF was to issue its ruling, Khan’s Golden Boy Promotions team withdrew its grievance. Then on Feb. 9, the sides officially announced the rematch during a news conference in the District as part of a media tour that included stops in England, where Khan resides, and Los Angeles.

It was following the March 19 news conference in Los Angeles that collections officers transported random urine samples from both fighters to a World Anti-Doping Agency-accredited Olympic laboratory on the campus of UCLA. Samples were split into an A and B sample, with the laboratory reporting Peterson’s positive A sample to VADA on April 12.

VADA informed Peterson of those findings, according to a letter to Schaefer from Nevada Athletic Commission executive director Keith Kizer, the following day and advised him of his right to have the B sample analyzed. Peterson’s team pursued that course, and that round of testing began on April 30. The results of the B sample substantiated the results of the A sample.

“Why it took from the 13th of April to the 30th to actually go and test the B sample is a mystery to me,” Schaefer said. “If I would have been informed about it, I certainly would have asked for expedited testing and not wait more than two weeks.”

Peterson subsequently submitted a random sample on April 13 that was found to be negative on May 2, Schaefer said.

Less than one week ago, Peterson held an open workout at the Headbangers Gym in Southeast Washington that trainer Barry Hunter operates. Peterson was upbeat and predictably confident at that time, going so far as to say Khan was not on the same level as himself.

“I feel the same way, a little bit better though,” Peterson said on Thursday when asked about his mood going into the rematch. “Just a little bit more at ease fighting on this level. Each time we do this, it’s getting easier and easier. I know exactly what to do in training camp, to prepare myself for a 12-round fight.”

Staff writer Barry Svrluga contributed to this report.