Cycling legend Lance Armstrong has hired a renowned doping scientist to prove his athletic greatness wasn't the result of performance-enhancing drugs, but the medical doctor who will be testing him says "there are no guarantees" that the results will be rock solid.

Don Catlin, who will be testing Armstrong's blood and urine as the cyclist trains for the Tour de France and other races next year, made the observation to the New York Daily News. "I think it's going to be as airtight as I can possibly make it," Catlin told the News in a story posted online today. "Anybody who tries to beat it will be a fool."

Catlin may be a master at picking up hard-to-detect drugs — it was he who discovered the formula for the synthetic steroid tetrahydrogestrinone (THG) at the center of the BALCO doping ring and a test to spot it — but doping is a constantly evolving science, with new drugs and supplements being tried all the time. Viagra, a legal drug for erectile dysfunction, is one of the latest athletic enhancers, raising the question of what positive test results for that pill might mean to the sports world.

Back in 2004 — a year after the BALCO scandal blew up — Catlin told Scientific American that he was already worrying about the next designer drug. Professional athletes in baseball, football, track and field and other sports have been linked to getting THG injections from BALCO, the San Francisco-area supplier of the drug.

"The fact that we finally characterized one is certainly no reason to celebrate," Catlin told the magazine then. "I'm much more worried about the next THG out there that we haven't found yet."

Catlin's lab, Anti-Doping Research, is now trying to find a urine test that can accurately pick up human growth hormone (hGH) — "a long-sought-after goal that Dr. Catlin believes is within reach," according to the company's Web site. Testing for another popular doping drug, the artificial hormone recombinant erythropoietin (r-EPO), hasn’t been perfected, either. Danish researchers reported this year that different labs get inconsistent results, and the test sometimes turns up false-positive results. Check out our list of some of the more popular drugs used by athletes.

(Image of Lance Armstrong at Grand Prix Midi Libre 2002/Wenn du Benutzer Hase)