Stress tests to detect blocked arteries in patients may miss more than half the cases of early heart disease, U.S. researchers reported on Tuesday.

Scientists found that 56 percent of patients who breezed through their stress tests in fact had significant hardening of the arteries needing treatment with diet, exercise, statin drugs, aspirin and other medications.

“Our findings demonstrate that a relatively high number of patients who had normal readings on their stress tests had a calcium score of greater than 100, a score that is accepted as implying the need for aggressive medical treatment,” said Dr. Daniel Berman, Director of Cardiac Imaging at Cedars-Sinai Medical Center in Los Angeles.

His team tested patients already considered at moderate risk of heart disease. “They usually had at least one risk factor,” Berman said in a telephone interview.

Most were men over 45 or women over 55, smokers, people with High cholesterol or high blood pressure, diabetes or a close relative with early heart disease, they reported in this week’s issue of the Journal of the American College of Cardiology.

The 1,195 patients in the study, who had no evidence of heart disease, underwent stress tests and a procedure called coronary calcium scanning.

A stress test uses a treadmill or an exercise bike to get a person’s heart working, and an electrocardiogram and blood pressure cuff to measure heart function.

The calcium test uses a quick burst of specialized X-rays called a computed tomography, or CT, to find evidence of plaques that block arteries. Among the 1,119 patients who had normal stress tests, 56 percent had calcium scores greater than 100, and 31 percent of patients had scores greater than 400, Berman found.

Calcium scores of zero are the “best” scores. Patients with calcium scores from 100 to 400 are at increased risk of cardiac events such as heart attacks, while patients with scores above 400 have the highest risk of a heart attack.

“Stress test results are very important for short-term events,” Berman said in a telephone interview. “The calcium test results are important for long-term events that will occur in five years to a decade or so.”

No one is certain how calcium is involved in blocking arteries, but as plaque is formed inside the blood vessel, calcium deposits form too. Berman said he has found calcium readings directly correlate with the amounts of blood-blocking plaque.

So patients with higher calcium scores “have a process going on in their arteries,” Berman said.

Because heart disease is the number one killer in most developed countries, Berman believes that most men over 45 and most women over 55, as well as smokers, people with High cholesterol and other risk factors, should have a calcium scan.

The problem is that most insurers will not pay for it, he said. “It’s about a $400 test,” Berman said. “Most third-party carriers are not paying for it.”