Relationship between kinetics of albumin-bound bilirubin and water-soluble urea in extracorporeal blood purification.

Background. The purpose of the study was to examine the relationship between urea and conjugated bilirubin kinetics during extracorporeal liver support (ELS) therapy and to determine the dose of therapy for urea and conjugated bilirubin as markers for water-soluble and protein-bound solutes, respectively. Methods. Kinetics of urea and bilirubin were described by standard two-compartment models with central clearance, constant intercompartment clearance: constant generation rate and constant volume. While the concentration of urea was assumed as equilibrated between compartments at the beginning of ELS therapy, the concentration of conjugated bilirubin between compartments was assumed to follow the partition of albumin between plasma and interstitial spaces. Treatment dose was calculated as removed solute mass and fractional solute removal. Results. Seven patients were studied during 15 treatments lasting at least 6 h. Bilirubin distribution volume of 14.8 +/- 5.4 L was not different from urea extracellular water volume of 15.0 +/- 2.8 L. The correspondence between models was used to predict the mass of bilirubin removed based on extracellular volume obtained from urea kinetics, average data from bilirubin kinetics, as well as selected treatment and patient information. The prediction of bilirubin mass removed based on this reduced information was not different from the mass of solute removed based on complete bilirubin kinetic analysis. Conclusions. The correspondence between kinetics of urea and conjugated bilirubin can be used to identify the bilirubin distribution volume from urea kinetic analysis. This information is then useful to estimate and predict the solute removal of conjugated bilirubin in ELS.