Transcellular disequilibrium and intradialytic catabolism reduce the reliability of urea kinetic formulas

To elucidate the limits of single-pool models as regard aberrations induced by urea transcompartmental disequilibrium during urea kinetics, mass balances and fractional clearances were studied, and original formulas developed to calculate urea clearance and distribution volume. In the early dialytic phases, aberrations were more evident, with low kinetic volume (VK) values and kinetic clearance (KK) values double those obtained by dialysate collection (KDC). Over the whole session, both VDC and VK were underestimated (9.9 and 8.2%), compared with anthropometric data (VA). In 3 patients, the comparison of VA and VDC, and the behavior of effective body water clearance (KE), agreed with the hypothesis of dialysis-induced catabolism. Both disequilibrium and hypercatabolism can affect the reliability of the single-pool urea kinetic model; because of overlap, their effects are difficult to separate, and became particularly important in high-efficiency dialysis. A modified model, using V and CtET as input is suggested in order to establish the most appropriate dialysis prescription for uremia therapy.