The article by Barnett and colleagues on the surgical treatment of asymptomatic carotid artery disease [1] deserves some clarification. First, the authors appear to misinterpret the failure to find statistically significant effects as implying the absence of an effect. The Asymptomatic Carotid Atherosclerosis Study (ACAS) was not designed to have good power to find differences in "disabling strokes" alone, to compare the treatments in women alone, or to evaluate differences in treatment effect between men and women.

Second, we agree with the authors that Doppler flow velocities reflect the cross-sectional area change of a vessel. It is precisely this measure, and not diameter, that is important for determining arterial pressure and flow. Nonetheless, Barnett and colleagues are wrong to imply that a "60% ACAS Doppler" stenosis is a measurement of area. All Doppler laboratories in the ACAS were validated to diameter measurements provided by arteriograms before being added to the study group. Thus, in the ACAS, 60%, 70%, and 80% Doppler stenoses are equivalent to 84%, 91%, and 96% cross-sectional area narrowing, respectively.

Third, the authors make much of the stroke risk determined by arteriographic linear decile measurements. We await their data showing good interobserver and intraobserver reliability in reading contrast angiograms to the nearest decile. Because the linear diameter of a normal carotid artery on digital minimized film is an average of 3 mm, the measurement of 60%, 70%, and 80% diameter stenosis would require exact measurements of 1.2-, 0.9-, and 0.6-mm minimal residual lumens, respectively. Given the vagaries of patient movement artifact, contrast-dye load and edge definition, projection angle, and intraluminal plaque irregularity, we believe that this task is daunting (perhaps impossible), even if a jeweler's loupe is used.

Finally, it is erroneous and misleading to imply that the results of ACAS are inconclusive. Peer reviewers (who critiqued the study before it began and periodically while it was in progress), the Data and Safety Monitoring Committee, and the unblinded Statistical Coordinating Center certified that the contrary was true. The Data Safety and Monitoring Committee stopped the study earlier than planned because the observed advantage of surgery was greater than we assumed when designing the study.