Dialyzer Ultrafiltration Coefficients: Comparison Between In Vitro and In Vivo Values

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Abstract
A simple, convenient method for the in vivo measurement of the ultrafiltration coefficient of hemodialyzers was described. The method was based on a scheme of isolated ultrafiltration, i.e., ultrafiltration without dialysate flow through the dialyzer. Results with this method indicated that it was more accurate than the conventional bed scale technique. Measurements on 3 different dialyzers demonstrated that the in vivo ultrafiltration coefficient was only between 1% and 10% lower than the corresponding in vitro value. This was in contrast to the rule of thumb used by some manufacturers that in vivo coefficients are 30% lower than in vitro values. The deviation of the in vivo value from the in vitro one seemed to be higher with higher dialyzer ultrafiltration coefficients. To estimate ultrafiltration rates in the clinical setting, the in vitro ultrafiltration coefficient should be used, transmembrane pressures being corrected for the colloid osmotic pressure of plasma proteins.