The U.S. Department of Agriculture quietly changed its policy for announcing the results of positive Mad Cow Disease tests.

In a statement posted on USDA's website but not announced to reporters, chief veterinarian John Clifford said instead of reporting positive or inconclusive results on a single round of testing -- as the agency has done in the past -- it will now wait until it has run two rounds of tests before informing the public.

"If either of the tests in the second run is reactive, (USDA) will announce this result as an inconclusive and will conduct confirmatory testing at National Veterinary Services Laboratories in Ames, Iowa," Clifford said.

Under the USDA's new surveillance plan launched June 1, two initial positives were reported that raised concern among the public and had negative impacts on the cattle futures markets, until they were ruled negative on follow-up testing.

The change in policy was made after the USDA completed proficiency evaluations of all of the labs participating in the new surveillance program and reviewed the results of the first 60 days of testing, Clifford said.