The Food and Drug Administration on Friday laid out a proposal to begin regulating laboratory medical tests, a multibillion-dollar industry that the agency says poses a growing risk to patients because of potentially inaccurate results. The proposed rule would end decades of regulatory ambiguity and formally bring thousands of tests performed in large laboratories under FDA oversight. FDA Commissioner Robert Califf said the change will help ensure tests used to diagnose cancer, heart disease and many other conditions are safe, accurate and reliable. “A growing number of clinical diagnostic tests are being offered as laboratory-developed tests without assurance that they work,” Califf said in a statement. He added that the agency has long worried that many tests offered by laboratories are not as accurate or reliable as those that undergo FDA review. Here’s a look at the history and background of the testing issue: What are laboratory-developed tests? Most Americans …