Estimation of delivered dialysis dose by on-line monitoring of the ultraviolet absorbance in the spent dialysate

Background. Several methods are available to determine Kt/V, from predialysis and postdialysis blood samples to using on-line dialysate urea monitors or to ionic dialysance using a conductivity method. The aim of this study is to compare Kt/V calculated from the slope of the logarithmic on-line ultraviolet (UV) absorbance measurements, blood urea Kt/V, dialysate urea KtIV, and KtIV from the Urea Monitor 1000 (UM; Baxter Healthcare Corp, Deerfield, IL). Methods: Thirteen uremic patients on chronic thrice-weekly hemodialysis therapy were included in the study. The method uses absorption of UV radiation by means of a spectrophotometric set-up. Measurements were performed on-line with the spectrophotometer connected to the fluid outlet of the dialysis machine; all spent dialysate passed through a specially designed cuvette for optical single-wavelength measurements. UV absorbance measurements were compared with those calculated using blood urea and dialysate urea, and, in a subset of treatments, the UM. Results: Equilibrated KtIV (eKt/V) obtained with UV absorbance (eKt/Va) was 1.19 +/- 0.23; blood urea (eKt/Vb), 1.30 +/- 0.20, and dialysate urea (eKt/Vd), 1.26 +/- 0.21, and KtIV in a subset measured by the UM (UM Kt/V) was 1.24 +/- 0.18. The difference between eKt/Vb and eKt/Va was 0.10 +/- 0.11, showing a variation similar to the difference between eKt/Vb and eKt/Vd (0.03 +/- 0.10) and in a subset between eKt/Vb and UM Kt/V (-0.02 +/- 0.11). Conclusion: The study suggests that urea KtIV can be estimated by on-line measurement of UV absorption in the spent dialysate.