MATTHEW PERRONE AP Business Writer WASHINGTON (AP) — U.S. health regulators have warned drugmaker Bayer over quality control issues at a plant that makes the key ingredient in Yaz and other popular birth control drugs. In a warning letter posted online Tuesday, the Food and Drug Administration said its inspectors uncovered testing problems at the company's plant in Berghamen, Germany, during a March visit. FDA inspectors said the company measured the quality of its drug ingredients based on an average of several samples, instead of reporting individual tests results. Bayer shipped eight drug batches to the U.S. that were tested using the method, which the FDA does not accept. The drug ingredients included drospirenone, which is used in the popular birth control pills Yaz and Yasmin. Those products were the company's top-selling pharmaceuticals last year, with global sales of $1.8 billion. After reviewing all drug batches shipped to the U.S. between 2007 and 2009, Bayer responded to the FDA in an April 7 letter that concluded "the quality of these batches was not affected." But the FDA responded in its own letter last month, disagreeing with Bayer's "rationale and conclusion." The agency has not advised patients to stop using the drugs and a Bayer spokeswoman on Tuesday said there have been no quality issues uncovered by a company assessment. "We have identified no safety and efficacy impact on our products in relation to the ... issues as cited by the FDA," Rose Talrico said in a company statement. The FDA has asked Bayer to provide a full list of all shipments to the U.S. that may have used the averaging method and provide a plan to prevent the problem from recurring. The FDA said it will consider delaying imports from the factory until Bayer addresses the warning letter. The FDA regularly issues warning letters to companies that do not follow regulations for manufacturing and marketing. The letters are not legally binding, but the agency can take companies to court if they are ignored. Earlier this summer, new FDA Commissioner Margaret Hamburg announced several changes designed to speed the pace of warning letters. They included a streamlined review process for sending the letters and shorter deadlines for companies to respond.