Supplementary Material for: Predictions of Serum Phosphate Concentration During CRRT Using a Steady State Mass Balance Model

Introduction: Hypophosphatemia is common during continuous renal replacement therapy (CRRT), but serum phosphate levels can potentially be maintained during treatment by either intravenous phosphate supplementation or addition of phosphate to renal replacement therapy (RRT) solutions. Methods: We developed a steady state phosphate mass balance model to assess the effects of CRRT dose on serum phosphate concentration when using both phosphate-free and phosphate-containing RRT solutions, with emphasis on low CRRT doses. Results: The model predicted that measurements of serum phosphate concentration prior to (initial) and during CRRT (final) together with clinical data on CRRT dose, treatment duration and phosphate supplementation can determine model patient parameters, that is, both the initial generation rate and clearance of phosphate prior to CRRT. Model parameters were then calculated from average patient data reported in several previous publications with a standard or high CRRT dose. Using representative model parameters for typical patients, predictions were then made of the effect of low CRRT dose on the change in serum phosphate levels after implementation of CRRT. The model predicted that CRRT at a low dose using phosphate-free RRT solutions will limit, but not eliminate, the incidence of hypophosphatemia. Further, the model predicted that CRRT at a low dose will have virtually no influence on the incidence of hyperphosphatemia when using phosphate-containing RRT solutions. Conclusions: This report identifies the clinical measurements for using the proposed model for individualizing the CRRT dose and RRT phosphate concentration to maintain serum phosphate concentrations in a desired range.