Since the initial formulation of the AHI, a large body of clinical and epidemiologic evidence has accumulated linking OSA severity to numerous clinical outcomes, including daytime sleepiness, impaired quality of life, motor vehicle accidents, incident hypertension, myocardial infarction, stroke, heart failure, diabetes, and all-cause mortality.,,, In most, if not all, available studies on the health significance of OSA to date, the AHI has been used as the primary exposure or independent variable and has been correlated with specific outcomes of interest. Moreover, interventional studies, which have examined the effects of positive airway pressure therapy, have shown that treatment of varying degrees of sleep apnea, as indexed by the AHI, is associated with favorable effects. Because of the consistency across studies in correlating the AHI with clinical sequelae, a strong foundation has formed supporting its use in characterizing OSA. Indeed, advocates of the AHI often cite the considerable evidence base of clinical outcomes to suggest that the AHI has criterion validity generalizable to several outcomes. Although the argument that AHI is a valid measure of disease activity because it predicts the presence or development of clinical outcomes is reasonable, it is not without limitations and should be, at best, considered a crude and imprecise metric of OSA.