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). The predictive accuracy may be used for both diagnostic and prognostic accuracy analyses; in the latter case, the comparison is to subsequent cardiovascular events. It is important to note that apparent test performance can be altered substantially by the pretest probability of IHD,52,80,81 making the accurate assessment of pretest probability and proper patient selection essential for diagnostic interpretation statements on IHD prevalence by test results. The positive predictive value of a test declines as the disease prevalence decreases in the population under study, whereas the negative predictive accuracy increases.82 Finally, the performance of noninvasive tests also varies in certain patient populations, Figure 7. The Ischemic Cascade. Reproduced with permission from Shaw et al.75 such as obese patients, the elderly, and women (Section 5.12), who often are underrepresented in clinical studies. Estimates of all test characteristics are subject to workup bias, also known as verification or posttest referral bias.81,83,84 This bias occurs when the results of stress testing are used to decide which patients undergo the standard reference proce- dure (invasive coronary angiography) to establish a definitive diagnosis of IHD (ie, patients with positive results on stress testing are referred for coronary angiography, whereas those with negative results are not). This bias has the effect of raising the measured sensitivity and lowering the measured specificity in relation to their true values. Mathematical corrections can be applied to estimate corrected values.84 86 Diagnostic testing is most valuable when the pretest probability of IHD is intermediate for example, when a 50-year-old man has atypical angina, and the probability of IHD is approximately 50% (Table 9). 