Modeling the postdialysis rebound: The reconciliation of current formulas

Three approaches are currently used in kinetic models (UKMs) to account for the postdialysis rebound in urea concentration, and thereby accurately mea sure the hemodialysis dose, KT/V (where K,T,V denote dialyzer clearance, di alysis duration, and urea distribution volume, respectively). The approach developed by Smye uses an intradialytic sample to predict the postdialysis equilibrium concentration, C-e, which is then used in a single pool UKM to give KT/V. A second approach developed by Tattersall introduces a patient c learance time, t(p). The true dialysis dose is then given by T/(T + t(p)) x apparent dose, and t(p) is estimated to be 36 minutes. The Daugirdas analy sis uses an empiric regression equation to give the true dose; KT/V)(true) from the single pool value, KT/V)(sp); KT/V)(true) = KT/V)(sp) - (36/T)(KT/ V)(sp) + 0.03. The analysis confirms the equivalence of all three formulas, which arises from the observation that during the later stages of dialysis , the urea concentration decreases as a single exponential. The formulas ar e independent of whether a flow or diffusion model is used to describe the kinetics of urea removal. The original analysis assumed constant volumes, b ut he effect of ultrafiltration volume u on C-e may be accounted for by mul tiplying by (1 + u/V). The Smye equation is more vulnerable to error in pra ctice, because small errors in the intradialytic sample give larger errors in the equilibrium concentration estimate, whereas dose estimates based on the Tattersall and Daugirdas equations are less affected by sampling errors . However, unlike the Smye approach, these two formulas would need adaptati on for use with other solutes. The advent of continuous urea monitoring sho uld permit more accurate, prospective estimates of equilibrium concentratio ns and dialysis dose.