To the editor: We commend Chertow et al. for taking an important step toward defining the "optimal dose of hemodialysis." However, several findings deserve clarification. First, the frequent-hemodialysis group received extra hours of weekly therapy,1 so that the increased hours, and not the frequency, may have led to the favorable results. Second, the patients in the frequent-hemodialysis group were, on average, 3 years younger than thepatients in the conventional-hemodialysis group. Could this trend toward a difference in age confound the outcomes, given that age is a significant covariate in this population? Third, the authors report increased complications of vascular access in the frequent-hemodialysis group. Although the time to the first intervention was shorter in these patients, there was no significant difference in the number of access events.
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