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Daugirdas JT, Burke MS, Balter P, Priester-Coary A, Majka T

Screening for extreme postdialysis urea rebound using the Smye method: Patients with access recirculation identified when a slow flow method is not used to draw the postdialysis blood

Am J Kidney Dis (Nov) 28:727-731 1996

During the past 2 years, there have been several papers and abstracts suggesting that perhaps post-dialysis urea rebound is predictable in the majority of patients, and that a simple offset based on the rate of dialysis (K/V) can be used to predict equilibrated Kt/V.

We started out this study to attempt to determine if there were patients out there who had a very large amount of post-dialysis rebound. With the cooperation of a large dialysis provider, we obtained an additional intradialytic sample on 3 consecutive months in several hundred dialysis patients. We knew from Smye's analysis that patients with a large postdialysis rebound also have a large "inbound"; i.e, such patients have urea sequestration somewhere in the body, and hence the serum BUN falls more abruptly than would be predicted by first order kinetics. Smye showed that one could use an intradialytic sample to find patients with high postdialysis rebound values. After screening 369 patients for 3 consecutive months, we found 27 who appeared to have a large "inbound", i.e, their intradialytic BUN sample was much lower than expected. We then were able to restudy 16 of these patients in a protocol which included actually measuring BUN at several time points after dialysis.

The results showed that most of these patients actually had a high degree of access recirculation. At the time of study, the clinics involved were not using a slow flow method to draw the post- dialysis BUN. They were simply stopping the pump to draw the post- dialysis sample. On restudy, when a slow flow method of drawing the post-sample was used, the excessive rebound disappeared, because the slow-flow postdialysis BUN (15 sec at Qb 50 ml/min) was much higher than the post sample drawn without a slow flow technique. Hence, rebound became similar to that predicted by the rate of dialysis or K/V (using the rate equation, delta Kt/V = 0.6 x K/V -0.03, as per Daugirdas and Schneditz, ASAIO J, 1995).

Comment: The findings of the study were two. (A) There probably are few patients undergoing outpatient dialysis with "huge" post-dialysis rebound values or very large "inbound" values once you exclude patients with access recirculation. (B) If a slow flow method is not used, about 6-7% of patients will have apparent URR and Kt/V values in the acceptable range, but will actually be quite underdialyzed. (John T. Daugirdas, M.D., University of Illinois at Chicago)