(Adds details, background) By Lewis Krauskopf Feb 10 (Reuters) - U.S. health regulators issued recommendations to manufacturers to ensure the safe production of the blood-clot prevention drug heparin, four years after a contamination involving the widely used product set off concerns about the global pharmaceutical supply chain. The draft guidance, posted on the website of the Food and Drug Administration, is designed to help manufacturers of active ingredients, finished products and others better control heparin to avoid contamination.