New guidelines issued Monday urge doctors to consider testing millions of Americans at moderate risk of heart disease for signs of inflammation in the bloodstream — a newly recognized cause of heart attacks.

Evidence has been building for several years that painless inflammation is a major trigger of heart trouble, worse than high cholesterol. But until now, doctors have been unsure how and when to look for the condition, which can be measured by a simple blood test.

The new recommendations, drawn up by the American Heart Assn. and the Centers for Disease Control and Prevention, are the first to propose an important role for inflammation testing as a way of judging whether people need aggressive treatment to protect their hearts.

The guidelines suggest limiting the testing to those already judged to be at 10 percent to 20 percent risk of heart disease over the next 10 years, based on such factors as age, high cholesterol and high blood pressure. This category is large, encompassing an estimated 40 percent of U.S. adults.

However, even in these patients, the test is considered optional and should be used only if it will help doctors decide whether they need treatment, which typically includes cholesterol-lowering drugs, better diets and weight loss.

The guidelines urge against testing people at very low risk, since those patients would probably not be put on treatment even if inflammation were found, as well as those already diagnosed with heart disease, since they should already be getting treatment.

“The guidelines are very much oriented toward coming up with numbers that would alter your or your patients’ behavior,” said Dr. Thomas Pearson of the University of Rochester, co-chairman of the committee that wrote the statement.

The inflammation comes from many sources — possibly including gum disease and lingering urinary infections — and triggers heart attacks by weakening the walls of blood vessels and making fatty buildups burst. It can be measured with a test that checks for C-reactive protein, or CRP, a chemical necessary for fighting injury and infection. The tests cost a few dollars to perform, and labs and hospitals charge between $10 and $120.