AQUEOUS SOLUTE CONCENTRATIONS AND EVALUATION OF MASS-TRANSPORT COEFFICIENTS IN PERITONEAL-DIALYSIS

The quantitative description of diffusive and convective mass transpor t of small solutes in peritoneal dialysis is dependent on accurate det ermination and appropriate expression of the concentration of investig ated substances. For small solutes which easily equilibrate between di alysate and plasma the solute concentration in plasma should be expres sed per volume of plasma water (aqueous concentration) and not per vol ume of whole plasma. Furthermore, the Donnan effect should be taken in to account for electrolytes if measured by flame photometry. The commo n practice of expressing solute concentration per volume of whole plas ma (plasma concentration) may result in substantial errors in calculat ed values of peritoneal transport parameters. To quantify these errors we compared plasma versus aqueous dialysate to plasma ratios (D/P), d iffusive mass transport coefficients (K(BD)), and sieving coefficients (S) for 28 6-h single-dwell studies using glucose 3.86% dialysis flui d. For all substances except glucose non-corrected plasma D/P overesti mated corrected aqueous D/P at 360 min by 2% (potassium and sodium) to 8% (creatinine) and, as assessed by the Pyle-Popovich model, non-corr ected K(BD) overestimated true K(BD) by 12% (potassium) to 41% (urea). Similar results were also obtained for K(BD) estimation using the Gar red model and K(BD) estimated during dialysate isovolaemia. The use of aqueous instead of plasma concentrations resulted in a substantial ch ange of S for urea but not for the other investigated solutes. These r esults emphasise the importance of expressing small solute concentrati ons per volume of plasma water and not per volume of whole plasma in c alculations of D/P ratios and mass transport coefficients.