WASHINGTON, Jan 16 (Reuters) - The U.S. Agriculture Department said on Tuesday it was investigating whether a corn sample found to be tainted with traces of StarLink genetically altered corn was taken from an American shipment destined for Japan.

Japan's Health, Labor and Welfare Ministry said late on Monday it found traces of the unapproved biotech corn variety in one of four samples sent on December 25 by U.S. grain exporters for food use. The same sample had tested negative in the United States.

The USDA said it reassured Japan that every effort was being made to prevent StarLink from seeping into U.S. corn exports. StarLink, made by Aventis SA(NYSE:AVE - news), has been at the heart of hundreds of food recalls due to concerns that it might cause allergic reactions.

This is the second time in less than a month Japan has found StarLink in U.S. corn samples. News of Japan's discovery sent Chicago Board of Trade corn prices lower on Tuesday. March corn futures closed down 3 at $2.16-1/4.

"We are trying to get more information on the testing procedures," said Tim Galvin, administrator for USDA's Foreign Agricultural Service.

Japan, the world's biggest buyer of American corn, agreed last month to begin testing for StarLink in U.S. corn shipments destined for the country's food supply and animal feed industry.

Under the U.S.-Japanese StarLink testing agreement, each week a USDA laboratory in Kansas tests three 400-kernel samples randomly chosen from the domestic corn harvest. These samples are then sent to Japan to verify its testing.

MISTAKEN IDENTITY?

USDA officials said Japan's positive sample might be from corn that was not scheduled for export to Japan.

But they also acknowledged that there was a slim possibility that StarLink contamination could be missed by its testing procedures. The genetic testing kit used by USDA officials at the Kansas laboratory has an accuracy of about 95 percent, they said.

Last month, Japan found traces of the StarLink protein Cry9c in a sample sent from the USDA laboratory. Japanese officials mistakenly believed the sample represented a U.S. shipment destined for Japan.

"I think there are some questions we need to clear up," said Steve Tanner, director of the Kansas laboratory opened last month by the Grains Inspection, Packers and Stockyards Administration (GIPSA). "We need to make sure we are doing all the same thing."

The USDA said it did not expect to revise its StarLink testing in U.S. grain exports despite Japan's discovery.

"I do not expect any changes in the program," said Isi Siddiqui, USDA's trade advisor. "It is working."

Masaki Sakai, Japan's counselor for agriculture to the United States, said Japan did not plan to ask for any revisions in the StarLink testing agreement.

"I think the situation will get better, not worse," Sakai said. "It takes a certain time to implement the protocol completely."

Strategic Diagnostics Inc. (NasdaqNM:SDIX - news) said there was no indication of any failures in its test kits for detecting gene-modified corn.

The company's Trait Bt9 kit was chosen by the USDA and Japan last November to detect StarLink in U.S. corn samples.

"We are extremely confident in our tests," said Art Coch, the company's chief operating officer.

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