“With some cases, you can win hands down,” he said. “The athlete will have a difficult time explaining the spikes and troughs of patterns in their blood. They won’t be able to say they used the wrong toothpaste, or that they got something from their dog. But the U.C.I. has to tread very carefully because this is such a new paradigm.”

For example, there would normally be a relatively constant level of hemoglobin in a rider’s profile out of competition, and the level would naturally decrease during a multistage competition, when the rider is exerting effort, said Michael Ashenden, another scientist on the U.C.I. panel. If a rider is doping, that level might not decrease, or could slightly increase, during that competition.

A conventional test can detect synthetic EPO in a rider’s system for a few days, Mr. Ashenden said, but a blood profile gives antidoping scientists an edge. “The signature that the rider used EPO will stay in the blood for much, much longer, maybe 5-10 days or longer, depending on the dosage,” he said.

Still, Mr. Ashenden said, the biological passport is not perfect. Riders who transfuse their own blood may not be caught because the swings in their blood values are not as dramatic as they can be with EPO use. He also said that the passport system would be successful for “about a year or so” until riders figure a way around it.

“Back in 2001, it would have decimated the sport because athletes were so careless and blasé about their blood doping,” said Mr. Ashenden, the project coordinator for an Australia-based research group, the Science and Industry Against Blood doping. “Now we’ll catch the least sophisticated ones. The others will be more careful.”

Antidoping scientists and officials, as well as other sports federations, are eager to see the outcome of a doping case based on a cyclist’s biological profile.

Mr. Parisotto said: “There’s a lot riding on this, more than just the credibility of the U.C.I. The philosophy of the passport and the future of the program will be on the line in the near future. They really must win their first case.”