Dialyzer ultrafiltration coefficients: comparison between in vitro and in vivo values

This study describes a simple, convenient method for the in vivo measurement of the ultrafiltration coefficient of hemodialyzers. The method is based on a scheme of isolated ultrafiltration, i.e., ultrafiltration without dialysate flow through the dialyzer. Results with this method indicate that it is more accurate than the conventional bed scale technique. Measurements on three different dialyzers demonstrate that the in vivo ultrafiltration coefficient is only between 1% and 10% lower than the corresponding in vitro value. This is 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 seems to be higher with higher dialyzer ultrafiltration coefficients. Based on these results, it is recommended that to estimate ultrafiltration rates in the clinical setting, the in vitro ultrafiltration coefficient be used, transmembrane pressures being corrected for the colloid osmotic pressure of plasma proteins.