The test to detect plasticizers from IV bags has been around for more than a year in antidoping, but is not yet validated for use, so an athlete could easily question its validity in court. Still, the test could be used in conjunction with other facts to build a doping case, antidoping experts said.

Image At a news conference in Spain last week, Alberto Contador discussed his positive test for clenbuterol but denied doping. Credit... Denis Doyle/Getty Images

“Even without a validated test, it could be looked at in a case-by-case basis,” Francesco Botré, the chief of the World Anti-Doping Agency-accredited laboratory in Rome, said. “If someone has a very, very high level of plasticizers in the urine, it would be hard for that athlete to explain how that happened if not for doping. If the level is lower, it obviously would make it much harder, but it would still be possible to prove.”

The second failed test, at the least, adds to Contador’s woes.

A day before the Tour’s decisive mountain stage, he tested positive for clenbuterol, a drug that boosts metabolism but can also increase aerobic capacity and the ability to process oxygen. He faces a two-year ban and loss of his Tour title if convicted of a doping offense.

Contador, who said he was tested eight times in the Tour’s final week, failed the test for clenbuterol on July 21, one day after the failed plasticizer test. Though the levels of the drug found in his urine were “very small,” according to the cycling union, doping rules say that any amount constitutes an offense.

Contador’s press agent, Jacinto Vidarte, said in a telephone interview on Monday that Contador “has done nothing illegal” and denies receiving any blood transfusions.

“There has been no official confirmation at all,” Vidarte said, regarding a news report last week that Contador had failed the test for the plasticizer.

That report, from the German state-run television station ARD, suggested that Contador’s positive test result for clenbuterol probably occurred because Contador transfused his own blood on that day, and that the stored blood he had used already had clenbuterol in it.