The measurement of endothelial dysfunction (ED) has importance in that it indicates the presence of coronary artery disease (Kuvin et al. in J Am Coll Cardiol 38(7):1843–1849, 2001) in addition to acting as a predictor of future adverse events (Halcox et al. in Circulation 106:653–658, 2002). Various tools, methods, and metrics exist that can provide an indicator of endothelial dysfunction. Given the signifi cance of ED, it is of utmost importance to fi nd a measurement technique that is reliable, while defi ning a metric providing a framework for an overall system that is practical, accurate, and repeatable. Success would provide a tool for the early detection of cardiovascular disease not only moving patients that are currently classifi ed as asymptomatic to symptomatic, but also providing a method to monitor the effi cacy of treatments.