Estimating urea volume in amputees on peritoneal dialysis by modified anthropometric formulas

Body composition determines body water content (the fraction body water/body weight). With developing obesity, body weight and body water increase, but body water content decreases. The anthropometric formulas for urea volume (body water) for Kt/V computations in nonamputated peritoneal dialysis subjects reflect this fundamental rule of body composition. However, the use of uncorrected anthropometric formulas in amputees provides body water content estimates inconsistent with the estimates of body composition obtained from nutritional assessment. Corrected estimates of urea volume can be obtained in three steps: (1) The non-amputated weight at the same body composition is computed by dividing the weight at the urea kinetic study (postamputation) by (1-the fractional weight loss from the amputation); (2) body water and body water content at this nonamputated weight are obtained from the appropriate anthropometric formula; (3) at the time of the urea kinetic study, post-amputation, body water is equal to the estimate of body water content obtained from step 2 times the body weight at the urea kinetic study. The corrected estimates of urea volume provide body water content values agreeing with the estimates from nutritional assessment.