The food poisoning test that ABC News Correspondent John Stossel used to allege that organic food "could kill you" cannot definitively prove any risk of food poisoning, according to a letter issued by the U.S. Department of Agriculture today.The letter, from a top USDA scientist, further undermines the credibility of a story on organic food safety by Stossel just hours before he is scheduled to apologize for errors in his report on ABC's "20/20 Friday.