Dialysis Research: Surface‐Area‐Normalized Kt/V: A Method of Rescaling Dialysis Dose to Body Surface Area—Implications for Different‐Size Patients by Gender

Abstract

Dialysis is measured as Kt/V, which scales the dose (Kt) to body water content (V). Scaling dialysis dose to body surface area (S_dub) has been advocated, but the implications of such rescaling have not been examined. We developed a method of rescaling measured Kt/V to S_dub and studied the effect of such alternative scaling on the minimum adequacy values that might then be applied in male and female patients of varying body size. We examined anthropometric estimates of V and S (Watson vs. Dubois estimates) in 1765 patients enrolled in the HEMO study after excluding patients with amputations. An S‐normalized target stdKt/V was defined, and an adequacy ratio (R) was computed for each patient as R = D/N where D = delivered stdKt/V (calculated using the Gotch–Leypoldt equation for stdKt/V) and N = the S‐normalized minimum target value. In the HEMO data set, we determined the extent to which baseline (prerandomization) stdKt/V values would have exceeded such an S‐based minimum target stdKt/V. The median V_wat:S_dub ratios were significantly higher in men (21.34) than in women (18.50). The average of these (20) was used to normalize the current suggested minimally adequate value (stdKt/V ≥ 2.0/week) to the S‐normalized target value (stdKt/S ≥ 40 L/M²), assuming that average modeled V = average anthropometric V. To achieve this S‐normalized target, the required single‐pool (sp) Kt/V was always higher in women than in men at any level of body size. For small patients (V_wat = 25L), required stdKt/V values were 2.05 and 2.21/week for men and women, respectively, corresponding to spKt/V values of 1.31 and 1.52/session. On the other hand, large (V_wat = 50L) male patients would need spKt/V values of only 1.0/session. Prerandomization baseline dialysis sessions in the HEMO study were found to meet such a new S‐based standard in almost all (766/773) men and in 885/992 women. An analysis of scaling dose to anthropometrically estimated liver size (L) showed similar gender ratios for V_wat:L and V_wat:S_dub, providing a potential physiologic explanation underpinning S‐based scaling. S‐based scaling of the dialysis dose would require considerably higher doses in small patients and in women, and would allow somewhat lower doses in larger male patients. Current dialysis practice would largely meet such an S‐based adequacy standard if the dose were normalized to a V_wat:S_dub ratio of 20.