Gay, swimmer Michael Phelps and sprinter Allyson Felix are among a dozen American athletes whose body chemistry is profiled using a series of *lo** and urine tests under a voluntary US Anti-Doping Agency (USADA) programme that Davies characterised as similar to that run by the IAAF.

Davies said the IAAF programme, like USADA's, uses so-called longitudinal tests in hopes of curtailing performance-enhancing drugs in the long term. Instead of testing against fixed, arbitrary numbers, the tests establish each athlete's body chemistry, then compare new tests to that baseline.

"It's what I'm doing to show my love for my sport," Gay said. "It comes with the territory. Past champions have tested positive. But an Olympic champion needs to carry himself clean, to be clean and to be able to prove he's clean."

The IOC has made a point of going after dopers at the Beijing Games, increasing its number of tests to about 4,500 - up from 3,600 for the Athens Games four years ago.

It is also doing 'target testing' of suspected dopers.

More than 50 athletes already are missing from the Games because of doping accusations.

"They are saying that they are doing, like, over 4,000 tests, but it's just very difficult," Powell said. Then he continued: "But hopefully they can ... make these Olympic Games very clean."