New models of phosphate kinetics in dialysis patients

During a dialytic treatment, plasma phosphate (Pi) has a peculiar time course. It reaches a nadir value approximately 90 minutes after the beginning of the treatment and then, despite the ongoing removal, it exhibits either a constant steady state or a slow rebound. In order to quantitatively describe this kinetics, the authors have proposed three different two-compartment models, in which the first compartment represents plasma and extracellular fluids and the second one intra-cellular fluids (IF); to explain the Pi time course, an endogenous release of phosphate must also be present. The models have been identified from a set of data (7 patients). The hypotheses underlying their structure and the identification results were discussed and one model was chosen as the best description of the system, in terms of both mathematical indices and physiological considerations. In this model, the endogenous production of phosphate enters the IF compartment and is dependent on the plasma concentration and on the difference from its initial value. This model was also capable of correctly predicting the qualitative time course of Pi also in the post dialysis period.