Figure 33

Definitions of flux, permeability, and efficiency. The urea value KoA, as conventionally defined in hemodialysis, is an estimate of the clearance of urea (a surrogate marker of low molecular weight uremic toxins) under conditions of infinite blood and dialysate flow rates. The following equation is used to calculate this value:


QbQd ln



Qb-Qd where Ko = mass transfer coefficient A = surface area Qb = blood flow rate Qd = dialysate flow rate ln = natural log

Kd = mean of blood and dialysate side urea clearance

As conventionally defined in hemodialysis, flux is the rate of water transfer across the hemodialysis membrane. Dissolved solutes are removed by convection (solvent drag effect).

Permeability is a measure of the clearance rate of molecules of middle molecular weight, sometimes defined using ^-microglobulin (molecular weight, 11,800 D) as the surrogate [3,4]. Dialyzers that permit ^-microglobulin clearance of over 20 mL/min under usual clinical flow and ultrafiltration conditions have been defined as high-permeability membrane dialyzers. Because of the general correlation between water flux and the clearance rate of molecules of middle molecular weight, the term high-flux membrane has been used commonly to denote high-permeability membrane.

Solute molecular weight, D
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