ORLANDO, FLA. — Reference norms for coronary artery calcium may be significantly lower than previously believed, Axel Schmermund, M.D., said at the annual meeting of the American College of Cardiology.
This new finding means that a fair number of patients deemed not to have a clinically significant amount of coronary artery calcium (CAC) on electron beam computed tomography (EBCT) for the detection of subclinical atherosclerosis may in truth have more than the average amount for their age—and thus may be at increased risk, explained Dr. Schmermund of the West German Heart Center at the University of Essen.
Until now, the reference standards applied throughout the world in using EBCT for the measurement of CAC as a means of stratifying coronary risk have relied principally on data from four large prospective U.S. studies. The studies involved subjects referred by their physicians or self-referred for EBCT, and hence are subject to several major types of bias.
Dr. Schmermund presented baseline data from the 4,259-subject Heinz Nixdorf RECALL (Risk Factors, Evaluation of Coronary Calcium and Lifestyle) study, the first large prospective study of CAC measurement in an unselected population.
“Our subjects were strictly unselected. We approached them, not the other way around,” he said.
The investigators found the distribution of CAC scores in their German population-based study differed significantly from that in the U.S. studies.
“Compared with American values, the Heinz Nixdorf values for the 50th percentile are all lower, at least in the higher age groups. There is an age shift of approximately 5 years,” Dr. Schmermund said.
This means, for example, that a 67-year-old man with a CAC Agatston score of 150 would be below the age-matched median using the widely cited American reference cohorts—but above the median value using the Heinz Nixdorf data as the reference norm.
Dr. Schmermund said it would be a mistake for American physicians to rely on German coronary artery calcium EBCT reference data to estimate the cardiovascular risk of U.S. patients, since the two populations differ in various ways. But the Heinz Nixdorf data raise a red flag regarding reliance on reference data that are based on referral populations. The good news, he added, is that soon American physicians will for the first time have CAC reference data obtained prospectively from an unselected, unbiased U.S. population, when the baseline data from the large prospective ongoing National Institutes of Health-sponsored Multiethnic Study of Atherosclerosis (MESA) are published.
An American observer singled out the Heinz Nixdorf study as a particularly important development in the noninvasive assessment of cardiovascular risk.
“The coronary artery calcium story has been going on for almost 15 years. One of my concerns about the technique is that the values that we've used to define normal and abnormal are always derived from highly biased cohorts of patients … who've been referred by their doctor for this test and who are also of a different socioeconomic status than most people. When you have an unselected prospective study like this one, it's amazing the differences you can see. I think Dr. Schmermund's study really gives us the yardstick by which the test can start to be even more useful than it's been,” commented James H. Stein, M.D., director of the vascular health screening program at the University of Wisconsin, Madison.