Bankhead MM, Toto RD, Star RA
Accuracy of urea removal estimated by kinetic models

Kidney Int (Sep) 48:785-793 1995

Dialysate-side urea kinetics is complex with currently available technology, but some previous work has suggested that dialysate side measurements are essential because blood sided measurements of Kt/V or solute reduction index (SRI) cannot be done reliably. Star et al compared 4 blood sided methods of urea kinetics (which do not involve any delayed post-dialysis blood samples) with total dialysate collection (which also involves measurement of the 1-hr post-dialysis BUN). Each of 8 patients were studied on 4 separate occasions. The four blood-sided methods were: formal single-pool UKM, a formal 2-pool (ECF-ICF) model (based on 45 and 90 min intradialytic samples), the Smye method (based on either a 45 min or a 90 min intradialytic sample), and a rate equation recently described by Daugirdas and Schneditz (ASAIO '95), which estimates dKt/V based on K/V, and requires only a pre and postdialysis sample. The principal findings were, that both the Smye method and the rate equation accurately predicted SRI. As the rate equation requires only a pre and post-dialysis sample, it has certain advantages over the Smye method. The implications of the study are, that perhaps total dialysate collection or dialysate sampling is not required in the majority of patients to accurately assess SRI.

Additional comments: Based on its good predictive power in both the study by Star and in analysis of pre-pilot and pilot NIH HEMO Study data (see ASN '95 abstract by Daugirdas et al), the rate equation is being used to target Kt/V in the full scale NIH HEMO trial. The rate equation may not work for patients taking vasodilators, esp. hydralazine or minoxidil or in patients with abnormally high or low cardiac output values (see ASN '95 abstract by TO George et al). With regard to the Smye method, the intradialytic and postdialysis samples should be drawn using exactly the same technique; i.e, if the 1 hr sample is drawn at full blood flow and the post sample using a 15 sec slow flow technique, results will not be accurate in patients with access recirculation (AR). If both samples are drawn at full blood flow in patients with AR, the Smye method may still give erroneous results when the degree of AR is not constant throughout dialysis (see Pflederer et al, Kidney Int. Sep '95, 48:832-837). When utilizing the Smye method, it is best to wait 15 sec (or 2 min) after slowing the pump to 50 ml/min when taking both the intradialytic and post-dialysis sample. (Daugirdas)

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Basic hemodialysis : Adequacy, prescription, urea kinetics