Validation of different methods to calculate Kt/V considering postdialysis rebound

BACKGROUND: The effect of increasing dialysis efficiency magnifies rebound urea and the error in Kt/V determinations from single pool urea kinetics. Several formulae have been developed to calculate Kt/V taking into account the rebound urea (Kt/Vr). Smye et al. proposed a method whereby the equilibrated BUN is predicted by an additional intradialytic urea sample (Kt/VrSmye). Daugirdas et al. proposed a method where a single pool Kt/V is modified according to the speed of dialysis to obtain a double pool Kt/V (Kt/VrDaug). Maduell et al. developed a method based on analysis of post-dialysis urea rebound whereby the Kt/Vr is predicted according to the single pool Kt/V and K/V (Kt/VrMad). DESIGN OF THE STUDY: We compared Kt/Vr estimated by these three formulae (Smye, Daugirdas, and Maduell) in 384 patients consisting of 211 males and 173 females, who received dialysis according to their regular protocols. Plasma urea was measured at the beginning, 90-100 min following the start of dialysis, the end, and 45 min post-dialysis. RESULTS: Post-dialysis rebound urea was 22.4 +/- 9.7%. Kt/V and Kt/Vr obtained with rea kinetic model Kt/V 1.184 +/- 0.22 and 0.984 +/- 0.20, respectively. These was a good correlation between Kt/Vr and the Smye formula (Kt/VrSmye = 0.956 +/- 0.21, r = 0.729, P < 0.001), and a better one for Daugirdas (Kt/VrDaug = 0.984 +/- 0.18, r = 0.931, P < 0.001), and Maduell formulae (Kt/VrMad = 0.980 +/- 0.18, r = 0.946, P < 0.001). Limits of agreement and percentage of error estimated according to Bland and Altman show that Kt/Vr estimated by Daugirdas and Maduell formulae could be used in place of the Kt/Vr. The degree of agreement with the Smye method is not clinically acceptable. CONCLUSION: Our results suggest that the use of a single pool Kt/V is not adequate to estimate the haemodialysis dose delivered and Kt/V taking rebound urea in consideration. Kt/Vr estimated by Daugirdas or Maduell formulae are a simple and accurate method for use in clinical practice.