Future directions in dialysis quantification

The influence of dialysis prescription on outcome is well established, and currently the amount of dialysis prescribed is based on small molecular wei ght toxin removal as represented by the clearance of urea. The "normalized dose of dialysis" (Kt/V-urea) concept is well established. Most techniques for dialysis quantification require that blood samples be taken at the begi nning and after the completion of dialysis. The postdialysis sample, howeve r, gives cause for concern because of the "rebound phenomenon" due to nonun iform distribution of urea among body compartments. Blood samples give "ind irect" measures of dialysis quantification. Thus direct urea concentration measurements in dialysate may be superior in urea kinetic modeling and thes e may be made "real time" during dialysis. It is with real-time monitoring that future advances in dialysis quantifica tion will take place. These will be of two types. The first will analyze bl ood water or dialysate samples for urea. content multiple times throughout the treatment; the second will assess the on-line clearance of urea using s urrogate molecules such as sodium chloride, the clearance being determined by conductivity measurements. On-line urea monitoring is based on the actio n of urease on urea in a water solution and measurement of the resultant am monium ions, which are measured directly by a specific electrode or indirec tly by conductivity changes. Differences in blood-side versus dialysate-sid e urea monitors exist which reflect the parameters they can provide, but wi th both, the standard urea kinetic measurements of Kt/V and nPCR (nPNA) are easily obtainable. A range of additional parameters can be derived from dialysate-side monitor ing such as "whole-body Kt/V" "pretreatment urea mass" and "whole-body urea clearance," which are worthy of future studies to determine their roles in adequacy assessment. Conductivity clearance measurements are made by exami ning the conductivity differences between dialysate inlet and outlet measur ed at two different dialysate inlet concentrations. This allows for the cal culation of the electrolyte (ionic) dialysance, which is equal to the "effe ctive" urea clearance, that is, the clearance that takes into account recir culation effects that reduce hemodialysis efficiency, The continuous readin g of effective ionic clearance wilt allow an average value for K to be obta ined for that dialysis, and hence the parameter K x t as an indication of d ialysis dose is easily and accurately obtained for every treatment. The con ductivity technology is cheap and rugged, and thus expanded use can be expe cted. Urea monitors have an inherent cost and require maintenance, and perh aps will remain researchers' tools for the present. The methodologies can c omplement each other; the addition of an accurate and independent value for K to dialysate based urea monitoring is like having simultaneous blood- an d dialysate-side monitoring, and allows further increase in measurable para meters.