Sieving coefficients

Here, L, is the ratio of pore to bulk diffusivity while x is fa sieving coefficient. The upper limit in the above integrals represents the fact that the spherical molecule cannot be any closer to the pore wall than its radius. Thus the values of K 1 and G must be determined. For A less than 0.4, the following were found as good... 

Electrical-charge effects can be further exploited by using charged membranes (as referred to above) to increase retention of all species with like polarity. It is important to mention that it may be possible to exploit electrostatic interactions even for solutes with similar isoelectrical points, due to different charge-pH profiles for the different species present. The membrane pore-size distribution also affects selectivity by altering the solute sieving coefficients locally. Narrow pore-size distributions, especially for electrically charged membranes, will impact very positively on membrane selectivity and overall performance. 

The extent to which a solute is carried in the ultrafiltrate across a membrane is characterized by its sieving coefficient (SC). An approximate equation for calculating sieving coefficients is... 

FIGURE 6.5 Relationship betwreen free fraction (/ ) and hemofiltration sieving coefficient (SC) for selected drugs. The line of identity (dashed line) indicates what would be expected if SC were equal to fu- (See text for further details.)... 

Ultrafiltration relies on the ability of a membrane to act as a selective barrier. One of the major properties of an ultrafiltration membrane is its ability to act as a sieve for macromolecular substances. The transmission of a particular solute through an ultrafiltration membrane can be expressed in several ways, a commonly used way being in terms of sieving coefficients. The intrinsic or real sieving coefficient (5j) is defined as... 

Most proteins would be expected to interact with the membrane to some extent. The intrinsic sieving coefficient of an interacting solute can be determined using Equation 18.5 [12] ... 

Due to concentration polarization, the solute concentration near the membrane surface tends to be higher than that in the bulk feed. Another sieving coefficient term that is commonly used is the apparent or observed sieving coefficient (S ) which is defined as... 

Membrane manufacturers prefer to use a parameter called the nominal molecular weight cut-off (NMWCO) or simply the molecular weight cut-off (MWCO) to specify the sieving properties of their membranes. For a given membrane this is defined as the molecular weight of a solute that has an apparent sieving coefficient of 0.1. Polymers, such as dextran or polyethylene glycol, are usually used to determine MWCO. 

Vincent FIFI, Vos MC, Akcahuseyin AE, et al. Drug clearance by continuous haemodiafiltration. Analysis of sieving coefficients and masstransfercoefficientsof diffusion. BloodPurlf. 1993 1 99-107... 

GSC glomerular sieving coefficient INR international normalized ratio... 

Protein Free Plasma Concen- Hr tration Diameter (kDa) (g/L) (nm) pi Glomerular Sieving Coefficient Filtered Load (mglLy Urinary Concen- tration (mg/L) % Reabsorbed... 

During CAVH/CVVH, drug removal primarily occurs via con-vection/ultraflltration (the passive transport of drug molecules at the concentration at which they exist in plasma water into the ultrafiltrate). The clearance of a drug by either of these methods is thus a function of the membrane permeability for the drug, which is called the sieving coefficient (SC) and the rate of ultrafiltrate formation (UFR). The SC can be calculated as ... 

TABLE 48—8. Predicted and Measured Sieving Coefficients of Selected Drugs... 

DFR, dialysate flow rate SC, sieving coefficient. From Joy et Mueller et and Bugge. ... 

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