If one considers their data in Table 1 and averages the mean relative mortality risk in the lowest two quintiles of body weight versus the highest two quintiles, one finds that the change in mortality risk is higher in the heavier patients when the warning level UF rate is exceeded. ( 4) suggest that a UF rate warning value on the basis of any of these body size measures is not optimal. I would argue that the data by Flythe et al. ( 4) have responded to this suggestion by pointing out that mortality risk is higher with higher UF rates in both men and women of different body sizes and that this is true whether body size is expressed as postdialysis weight, body mass index, or body surface area ( Table 1). Daugirdas and Schneditz ( 3) have argued that, on the basis of physiologic measurements such as those related to blood volume, the UF rate might be better scaled to body surface area than to body weight. On the basis of observational data, guidelines and quality assurance metrics have been proposed, suggesting a maximum allowable UF rate, such as 13 ml/kg per hour, although it has been recognized that the higher mortality risk associated with UF rate might be more or less continuous. There have been no randomized trials examining this issue and very few interventional trials. These associations have been derived from observational dataset reports, with all of their known shortcomings. It is now well established that higher ultrafiltration (UF) rates during a hemodialysis session are associated with a higher propensity to intradialytic hypotension, cardiac stunning, and mortality ( 1, 2).

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