Don't screen those at low risk for CHD

Get off the treadmill, the tests don't work.
The ghost of Framingham

By Owen Dyer

New US guidelines on screening for the prediction of coronary heart disease (CHD) events in low-risk adults have recommended against three techniques now in common use — exercise treadmill testing (ETT), resting electrocardiograms (ECG), and electron beam computerized tomography scanning for coronary calcium. These diagnostic tools are also deemed unsuitable in screening for the presence of severe coronary artery stenosis in low-risk adults. The guidelines, drawn up by the US Preventive Services Task Force (USPSTF), were published in the current issue of the journal Annals of Internal Medicine.

These three techniques, used to identify heart disease in low-risk adults or to find severe coronary artery stenosis, receive a grade D recommendation from the USPSTF. This is a recommendation against routine use in asymptomatic patients, meaning that the USPSTF found "at least fair evidence that the service is ineffective or that harms outweigh benefits."

In predicting CHD events or detecting stenosis in adults at high risk for heart disease, the USPSTF issued a Grade I recommendation for the three techniques, which amounts to neither for nor against. These recommendations are issued when "evidence that the service is effective is lacking, of poor quality or conflicting, and the balance of benefits and harms cannot be determined."

The three tests are all judged to have poor to fair accuracy in predicting CHD events.

The problems begin, said the USPSTF, with the relatively poor sensitivity and specificity of these tests. The guidelines argued that "the consequences of false-positive tests may potentially outweigh the benefits of screening. False-positive tests are common among asymptomatic adults, especially women, and may lead to unnecessary diagnostic testing, overtreatment and labelling." They are backed up by a study in the American Heart Journal, which found that among asymptomatic patients who tested positive for stenosis on ETT, only 29% were actually found to have stenosis upon angiography.

Moreover, the guidelines said, "screening could also result in false-negative results. A negative test does not rule out the presence of severe coronary artery stenosis or a future CHD event."

Another problem is the lack of research which judges these diagnostic tools in terms of their effect on clinical outcomes. For example, the USPSTF suspected that although major ECG abnormalities may be more prevalent in black men than in white men, these abnormalities may not confer the same risk for CHD death in black men.

These new guidelines are part of an ongoing reassessment in cardiology of the problem of the low-risk patient. These are people who, based on calculations such as the Framingham equations, are judged to have a less than 10% risk of suffering a CHD event in the next 10 years. But the conundrum is that since they make up the vast majority of patients, they still account for the great majority of CHD events. This is inevitable when so many acute CHD events result from sudden occlusion of a previously unobstructed artery segment, said the researchers.

Lately, the universal applicability of the Framingham equations themselves has been questioned. These latest findings suggested that a reliable way of identifying asymptomatic patients who will benefit from preventive treatment is still some way off.