Pitfalls of in vivo dialyzer clearance measurement

Dialyzer small-molecule clearance measurements are commonly made to help identify the cause of inadequate dialysis prescriptions, to determine the efficacy of reuse procedures, or to choose between different types of dialyzers. Clearance measurements can be blood-side- or dialysate-side-based. While blood-side clearance measurement is the classical technique, it suffers from several serious flaws that decrease its accuracy. Chief among these are the inability to accurately measure the blood flow rate and the difficulty in accounting for the presence of nonaqueous components in the blood. Using a dialysate-based clearance measurement technique overcomes these problems for most solutes, provided appropriate guidelines are followed. This article reviews the theory behind both blood- and dialysate-side techniques as well as discussing the practical application of that theory to clearance measurement.