We agree with Kutner and Zhang that the assignment of frailty depends on the particular criteria used and operational measures applied. We are also fully aware of the difference between self-reported physical function (PF) and physical performance as measured by grip strength or gait speed. However, when not all measures in the original definition of frailty by Fried et al1 are available, we believe that any substitute measures should correlate well with the original criteria and that the adapted definition should predict adverse health outcomes, a key component in the conceptual framework of frailty. Johansen et al2 have previously shown that the PF scale of the Medical Outcomes Study Short Form 36-Item Health Survey (SF-36) correlated strongly with physical performance as measured by gait speed in patients with end-stage renal disease (ESRD). In the Women's Health Initiative clinical trial, there was also good correlation between the PF scale of the SF-36 and physical performance as measured by gait speed and grip strength.3 In addition, the adaptation of the original definition of frailty by Fried et al1 to a questionnaire-based assessment has been adopted by others in studying both non-ESRD and ESRD populations and has been shown to be associated with adverse health outcomes including mortality.3,4 Thus, we believe that our adaptation of Fried and colleagues' original frailty definition is valid because the substitute measures correlate with the original criteria, and the final modification remains a valid predictor of mortality and morbidity in patients with ESRD.
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