"I believe it's the first time in the United States a race of this magnitude has had a full field tested for EPO," said John Veitch, chief steward for the Kentucky Horse Racing Authority and a former trainer.

The surprise screening for blood-doping agents such as erythropoietin, or EPO, and darbepoietin was conducted Wednesday.

Veitch said the decision to do pre-race testing was not motivated by suspicions of wrongdoing but to ensure enforcement of the state's medication rules.

Most testing for performance-enhancing drugs has been done after the races. Researchers now believe that drugs can be detected in a horse's system only three to five days after being ingested, but the effects last longer.

"The tests are reasonably new, and it helps us maintain the integrity of the sport," Veitch said.