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Abstract: FR-PO480

Plasma Viscosity Can Explain the Marked Reduction in Dialyzer Mass Transfer Area Coefficient for Urea and Other Solutes In Vivo Compared with In Vitro Values

Session Information

Category: Dialysis

  • 701 Dialysis: Hemodialysis and Frequent Dialysis


  • Schneditz, Daniel, Medical University of Graz, Graz, Austria
  • Daugirdas, John T., University of Illinois College of Medicine, Chicago, Illinois, United States

The dialyzer area membrane transfer coefficient product (K0A) characterizes the diffusive performance of a dialyzer and is one of the key components to prescribe a dialysis dose. However, in-vivo dialyzer K0A for urea is always much smaller and only 50 to 60% of in-vitro K0A tabulated in dialyzer manufacturer sheets and typically obtained from clearance measurements using crystalloid water solutions. The reason for this reduction has not been clearly determined. We hypothesized that the known effect of viscosity on solute diffusivity might partially or fully account for this reduction.


In-vitro dialyzer clearance of urea and glucose was measured in low- and high-flux dialyzers under different operating conditions using crystalloid solutions as well as bovine blood with different hematocrit and plasma viscosity. Viscosity of plasma, blood, and aqueous solutions was measured at 37°C. Diffusivity and relative K0A values were computed for each solute under these different conditions.


Relative K0A was negatively correlated to relative plasma viscosity (p<0.001, r2=0.38) and solute diffusivity (p<0.001, r2=0.48) for urea and glucose (Fig. 1). Plasma was 1.84±0.31 times more viscous compared to crystalloid test solutions with a viscosity of 0.72 mPa.s, suggesting a correction multiplier of 0.54 (=1/1.84) for in-vivo solute diffusivity and K0A relative to the in-vitro value. The average multiplier based on individual measurements was 0.67±0.09.


The known effect of viscosity on solute diffusivity is sufficient to explain the reduction of dialyzer K0A for urea and glucose in-vivo compared to in-vitro measurements. The residual scatter in the K0A to viscosity relationship suggest that additional mechanisms may be operative.

Fig. 1 Relative K0A for urea (closed circles) and glucose (open circles) as function of relative plasma viscosity (left panel) or solute diffusivity (right panel). Dashed lines show the linear regressions for both urea and glucose.