This week, cardiologists learned that a new online calculator meant to help them determine a patient’s suitability for cholesterol treatment was flawed, doubling the estimated risk of heart attack or stroke for the average patient. But fixing it would not be easy, because it is based on older data, and heart attack and stroke rates today are much lower than in decades past, meaning that people are at less risk than might be expected from historical extrapolations.

Yet the outdated risk figures are the only ones available for researchers to use as assessment tools, cardiologists say, and that raises real problems for the new risk calculator, which the American Heart Association and the American College of Cardiology posted online last week as part of a radical new set of guidelines for treating high cholesterol. The guidelines, which are supposed to shape the way doctors prescribe cholesterol-lowering statins, recommend looking beyond a patient’s cholesterol readings.

“The disease of atherosclerosis is changing before our eyes,” said Dr. Peter Libby, the chief of cardiovascular medicine at Brigham and Women’s Hospital in Boston. The reasons for the changes, he said, are only partly understood.

The problem of using longitudinal health studies from previous decades to assess health risks today arose unexpectedly last weekend at the annual meeting of the American Heart Association. Two Harvard researchers, Dr. Paul M. Ridker and Dr. Nancy Cook, revealed that the new calculator released with fanfare last week exaggerated the true risk of a heart attack or stroke by an average of 100 percent. Moreover, they said, the committee that developed the calculator knew that the online tool was inaccurate yet told doctors to use it in deciding whom to treat.