How low-risk is a coronary calcium score of zero? The importance of conditional probability.

Coronary artery calcium (CAC) scoring by computed tomography (CT) has been the subject of intense interest and critical scrutiny since it was first reported as a clinical tool in 1990.1 With improvements in study design, greater availability of coronary CT scanners, and increased attention to the posttest prognosis of patient samples and asymptomatic individuals who have undergone coronary CT, CAC measurement is now considered a potentially useful test for improving coronary risk assessment in selected intermediate-risk asymptomatic patients in whom high CAC scores signify increased cardiovascular risk beyond that predicted by conventional cardiovascular risk factors alone.2 Article p 1693 At the other end of the spectrum, does a very low CAC score signify very low risk? An American Heart Association writing group3 stated that a CAC score of zero (CAC=0; ie, no calcified plaque detected) indicated 1) that the presence of atherosclerotic plaque, including unstable or vulnerable plaque, was highly unlikely; 2) that the presence of significant luminal obstructive disease was highly unlikely (negative predictive value on the order of 95% to 99%); and 3) that the risk of a cardiovascular event in the next 2 to 5 years was quite low (0.1 per 100 person-years). In addition, at least 1 early study suggested that CAC=0 might be useful in the emergency room setting as a tool to rule out myocardial ischemia in symptomatic patients.4 A recent review article5 suggested the same conclusions. However, as pointed out by a different …