Mass Balance Index: An Index for Adequacy of Dialysis and Nutrition

Abstract

Determining adequacy of dialysis has remained a problem for the nephrologist despite the results of the National Cooperative Dialysis Study published more than 20 years ago. Urea Kinetics Modelling (UKM) which requires computer data entry is time-consuming for the dialysis staff but is the only method that has been rigorously studied. Furthermore, it is unclear today what value of Kt/V represents ideal dialysis; the technique is subject to a number of errors associated with estimation of dialyser clearance (K) and volume of distribution of urea (V) but it is useful for calculating protein catabolic rate (PCR). Methods that use urea reduction ratios (URR) is widely used because it is simpler but not always accurate and suffer from an inability to calculate PCR. Direct dialysis quantification (DDQ) can overcome a number of these problems but it is too cumbersome for routine use. Simpler methods to determine dialysateside kinetics have the advantage of solving a number of these problems and also facilitate the calculation of PCR to determine the patient's nutritional state. In our study, we have demonstrated that by taking two dialysate samples at the beginning and at the end of dialysis (2-DSM), it is possible to determine total urea removal (TUR) which is equivalent to DDQ. By taking blood samples after dialysis and before the next dialysis, it is possible to calculate the total urea generated (TUG). The ratio of TUR/TUG will provide an index of dialysis which places emphasis on removal of solute that has accumulated in the inter-dialytic interval thus re-establishing a state of equilibrium. We refer to this index as the Mass Balance Index (MBI). The MBI is also useful in helping to identify those patients whose PCR is inadequate since the mean MBI for patients with an nPCR <0.8 was 0.93 ± 0.03 vs 1.08 ± 0.02 in those with a PCR >0.8. In these two groups of patients the Kt/V was not significantly different, 1.49 ± 0.07 vs 1.53 ± 0.06, p -0.64. We suggest that the emphasis for adequacy of dialysis should shift away from Kt/V to maintaining a state of equilibrium by removing the solutes that accumulate between dialysis and by identifying those patients with an inadequate PCR.