SummaryBackground and objectives The kinetics of plasma phosphorus (inorganic phosphorus or phosphate) during hemodialysis treatments cannot be explained by conventional one-or two-compartment models; previous approaches have been limited by assuming that the distribution of phosphorus is confined to classical intracellular and extracellular fluid compartments. In this study a novel pseudo one-compartment model, including phosphorus mobilization from a large second compartment, was proposed and evaluated.Design, setting, participants, & measurements Clinical data were obtained during a crossover study where 22 chronic hemodialysis patients underwent both short (2-hour) and conventional (4-hour) hemodialysis sessions. The model estimated two patient-specific parameters, phosphorus mobilization clearance and phosphorus central distribution volume, by fitting frequent intradialytic and postdialytic plasma phosphorus concentrations using nonlinear regression.Results Phosphorus mobilization clearances varied among patients (45 to 208 ml/min), but estimates during short (98 Ϯ 44 ml/min, mean Ϯ SD) and conventional (99 Ϯ 47 ml/min) sessions were not different (P ϭ 0.74) and correlated with each other (concordance correlation coefficient c of 0.85). Phosphorus central distribution volumes for each patient (short: 11.0 Ϯ 4.2 L and conventional: 11.9 Ϯ 3.8 L) were also correlated ( c of 0.45). ConclusionsThe reproducibility of patient-specific parameters during short and conventional hemodialysis treatments suggests that a pseudo one-compartment model is robust and can describe plasma phosphorus kinetics under conditions of clinical interest.