Urea, creatinine and phosphate kinetic modeling during dialysis: application to pediatric hemodialysis.

The kinetics of urea, creatinine and phosphate removal during dialysis were investigated in pediatric patients using a two-pool model taking into account fluid shifts and mass transfer between the two compartments. It is found that even urea must be described by a two-pool model since it presents a post dialysis rebound due to equilibration between the two compartments. Phosphate plasma concentration drops very sharply during the first hour of dialysis and rises rapidly during the rebound period. This pattern cannot be accounted for by the classical two-pool model with constant generation rate and mass transfer coefficients, but corresponds to a large time-dependent phosphate influx from the intracellular compartment in which phosphate is generated by biochemical reactions or liberated from the bones. This influx was calculated for four patients representing 8 dialysis sessions and was found to reach a plateau after 90 minutes of dialysis, dropping rapidly during the rebound period.