Comparison of hemodialysis urea clearance using spent dialysate and Kt/Vurea equations

Abstract

IntroductionDialysis adequacy is traditionally calculated from pre‐ and post‐hemodialysis session serum urea concentrations and expressed as the urea reduction ratio, or Kt/Vurea. However, with increasing hemodiafiltration usage, we wished to determine whether there were any differences between standard Kt/Vurea equations and directly measured spent dialysate urea clearance.MethodsUrea clearance was measured from collected effluent dialysate and compared with various other methods of Kt/Vurea calculation, including change in total body urea from measuring pre‐ and post‐total body water with bioimpedance and the Watson equation, by standard Kt/V equations, and online clearance measurements using effective ionic dialysance (OLC).ResultsWe compared urea clearance in 41 patients, 56.1% male, mean age 69.3 ± 12.6 years with 87.8% treated by hemodiafiltration. Reduction in total body urea was greater when estimating changes in total body urea, compared to measured dialysate losses of 58.4% (48.5–67.6) vs 71.6% (62.1–78), p < 0.01. Sessional urea clearance (Kt/Vurea) was greater using the online Solute‐Solver program compared to OLC, median 1.45(1.13–1.75) vs 1.2 (0.93–1.4), and 2nd generation Kt/V equations 1.3 (1.02–1.66), p < 0.01, but not different from estimated total body urea clearance 1.36 (1.15–1.73) and dialysate clearance 1.36 (1.07–1.76). The mean bias compared to the Solute‐Solver program was greatest with OLC (−0.25), compared to second‐generation equations (−0.02), estimated total body clearance (−0.02) and measured dialysate clearance (−0.01).ConclusionThis study demonstrated that the result from equations estimating urea clearance indirectly from pre‐ and postblood samples from hemo‐ and hemodiafiltration treatments was highly correlated with direct measurements of dialysate urea clearance.