Usefulness of Assessing Hemodialysis Adequacy Using the Stop Dialysate Flow (SDF) Method

Cho, Min Seok; Jeong, Gyun Ho; Lee, Yoon Chul; Cho, Young Wook; Kim, Hyun Soo; Ma, Seong Kwon; Kim, Soo Wan; Kim, Nam Ho; Choi, Ki Chul

Abstract

BACKGROUND
K/DOQI guidelines recommend the slow flow method as a standardized method of postdialysis blood sampling for measuring hemodialysis adequacy. However, it is not easy to adopt this method when working in busy renal units where it is often difficult to obtain repeated samples exactly at the specified time. The stop dialysis flow (SDF) method recommended by the Scottish Renal Association since 1998 has the advantage of involving 2 steps only: (1) switch off dialysate flow at the end of hemodialysis without altering the blood pump speed and (2) take a blood sample after 5 minutes from the arterial or venous port. However, there are some limitations to SDF mthod in that it does not allow for tissue rebound after the first 5 minutes postdialysis and cannot be used directly to calculate equilibrated Kt/V (Kt/Veq) using either a 30-minute postdialysis sample. We derived a formula that uses a 5-minute postdialysis BUN sample using the SDF method to estimate the BUN concentration at 30 minutes and investigated if it is useful to assess hemodialysis adequacy using this method. METHODS: A total of 51 patients who had been undergoing hemodialysis in Chonnam National University Hospital and had agreed in joining this study were involved. Patients were randomly selected to 2 groups. Blood samples were obtained immediately before dialysis and at 0, 5, and 30 minutes postdialysis. We calculated the linear relationship between the 5-minute and 30-minute postdialysis samples in group A patients (n=25) and validated this equation using the data from the other group B patients (n= 26). We predicted what the 30-minute BUN concentration would be using the measured value of BUN at 5 minutes and compared directly the value of our estimated 30-minute BUN with the measured 30- minute BUN. RESULTS: There was a tight linear correlation (R2=0.993, p<0.05), between measured 5-minute postdialysis BUN concentrations and measured 30-minute postdialysis BUN concentrations in group A patients. This relationship is described by the linear regression equation: 30-minute BUN concentration=1.05x(5-minute BUN concentraion)+1.04. We used this equation to estimate the 30-minute BUN concentration in group B patients based on the 5-minute postdialysis BUN sample from these patients. And there was a close correlation between estimated and measured 30-minute postdialysis BUN concentration (R2= 0.989, p<0.05). The sensitivity, specificity, positive, and negative predictive values of this equation were high when used to estimate 30-minute urea reduction ratio (URR) greater than 65% (88.9%, 100%, 100 %, and 94.4%, respectively) and 30-minute Kt/Vsp greater than 1.2 (100%, 100%, 100%, 100%, respectively). CONCLUSION: We could estimate 30-minute postdialysis BUN concentration, 30-minute Kt/V, and 30-minute URR exactly using SDF method and linear regression equation derived in this study. The advantage of involving 2 steps only makes SDF method a useful tool in assessing hemodialysis adequacy.