The Guidance suggests that there is no need for further testing in those with an estimated likelihood ,10%. In those with an estimated likelihood of CAD of 10% to 29%, the National Institute for Clinical Excellence document recommends beginning with CT coro- nary artery calcium (CAC) scoring as the first-line diagnostic investigation, whereas the present SIHD guideline provides a Class IIb recommendation for several reasons, as outlined in Section 2.2.4.2. 2.2.1.1. Assessing Diagnostic Test Characteristics A hierarchy of diagnostic test evidence has been proposed by Fryback and Thornbury78 and ranges from evidence on technical quality (level 1) through test accuracy (sensitivity and specificity associated with test interpretation), to changes in diagnostic thinking, effect on patient management, and patient outcomes, to societal costs and benefits (level 6). A similar framework has been proposed for biomarkers by Hlatky et al.79 In practice, although knowledge of the effect of diagnostic testing on outcomes would be highly desirable, the vast majority of available evidence is on diagnostic or prognostic accuracy. Therefore, this information most com- monly is used to compare test performance. Diagnostic accuracy is commonly represented by the terms sensitivity and specificity, which are calculated by comparing test results to the gold standard of the results of invasive coronary angiography. The sensitivity of any noninvasive test to diagnose SIHD expresses the frequency that a patient with angiographic IHD will have a positive test result, whereas the specificity measures the frequency that a patient without IHD will have a negative result. In addition, predictive accuracy represents the frequency that a patient with a positive test does have IHD (positive predictive value) or that a patient with a negative test truly does not have IHD (negative predictive value). 