Because the presence of signif- icant calcification often can preclude the accurate assessment of lesion severity or cause a false positive study, CCTA should not be performed in patients who have known exten- sive calcification or a high risk of CAD. 2.2.4.2. CAC Scoring CT also provides measurement of a CAC score, calculated as the product of the CAC area by maximal plaque density (in Hounsfield units).217 The CAC score frequently has been applied for risk assessment in asymptomatic individuals,5 and it also has been used to predict the presence of high-grade coronary stenosis as the cause of chest pain in symptomatic patients. When the data from 2 large multicenter registries, including a total of 3615 symptomatic patients, were com- bined, the estimated diagnostic sensitivity for the CAC score to predict obstructive CAD on invasive angiography was 85%, with a specificity of 75%.218 In a recent meta-analysis of 18 studies, which included 10 355 symptomatic patients, the presence of nonzero CAC score had a pooled sensitivity and specificity of 98% and 40%, respectively, for detection of significant CAD on invasive coronary angiography.174 Although the diagnostic sensitivity of CAC to detect obstructive CAD is fairly high, the frequency of false nega- tive exams (ie, significant CAD in the absence of CAC) is not well established. 