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

Abstract

To look for patients with extreme urea rebound, we drew intradialytic samples one third of the way into dialysis during routine modeling for 3 months. The samples taken postdialysis were obtained after stopping the blood pump, without any slow flow period. Using the Smye equations, the intradialytic urea level was used to predict urea rebound, expressed as Kt/V-equilibrated minus Kt/V-single pool (ΔKt/V). Results were averaged for the 3-month period in 369 patients. Mean estimated ΔKt/V was −0.20 ± 0.13, which was similar to but slightly higher than the predicted value (−0.6 × K/V ± 0.03) of −0.19 ± 0.04. In 27 patients, extreme rebound (mean ΔKt/V < − 0.40) was found. Sixteen of these patients consented to further study, but only after access revision in four patients. In these patients, additional slow flow samples after 15 seconds and 2 minutes of slow flow, respectively, were drawn one third of the way into dialysis and postdialysis, and a sample was drawn 30 minutes after dialysis. On restudy, postdialysis rebound was still high with full flow samples ΔKtN = −0.40 ± 25, but was much lower (−0.18 ± 0.07) and similar to predicted rebound (−0.19 ± 0.05; P = NS) when based on 15-second slow flow samples. Eight of the 16 had marked (>15%) access recirculation by urea sampling, and ΔKtN based on full flow post samples correlated with access recirculation ( r = −0.91). The results suggest that the Smye method is valuable for identifying patients with aberrantly large postdialysis rebound values. When the postdialysis samples are drawn without an antecedent slow flow period, most patients with extreme rebound values turn out to have marked access recirculation.