WASHINGTON (CNN) - The U.S. Department of Agriculture is investigating an inconclusive test result for mad cow disease, the agency announced Thursday.

Shares of McDonald's (Research) fell nearly 1 percent on news of the test.

"The inconclusive test result does not mean we have found another case of BSE (bovine spongiform encephalopathy) in this country," the agency's written statement said.

"Inconclusive results are a normal component of screening tests, which are designed to be extremely sensitive so they will detect any sample that could possibly be positive."

The agency said it is sending tissue samples to a national laboratory for "confirmatory testing."

The animal in question never entered the food or feed chain, the USDA said.