Estimating total urea removal and protein catabolic rate by monitoring UV absorbance in spent dialysate

Background. Dialysate-based, on-line measurements of Kt/V and protein catabolic rate (PCR) in dialysis patients have been considered more accurate compared with measurements on the blood side during dialysis. The primary aim of this study was to compare total removed urea (TRU) and PCR, normalized to body weight (nPCRw), obtained by three dialysate-based methods: (i) on-line ultraviolet (UV) absorbance of the spent dialysate; (ii) total dialysate collection (TDC), as reference method; and (iii) Urea Monitor 1000 (UM) from Baxter Healthcare Corp. Methods. We studied 10 uraemic patients on chronic, thrice-weekly haemodialysis. We made absorption measurements ( UV radiation) on-line with a spectrophotometer connected to the fluid outlet of the dialysis machine, with all spent dialysate passing through an optical cuvette for single-wavelength measurements. UV absorbance measurements were compared with TDC and the UM. Results. nPCRw obtained with UV absorbance was 0.82 +/- 0.17, that from TDC 0.81 +/- 0.18, and that measured by UM 0.87 +/- 0.18, which was significantly higher than the results of the other methods. The difference between nPCRw calculated by TDC and by UM was -0.05 +/- 0.06, showing a slightly lower SD than the difference between nPCRw by TDC and UV absorbance, -0.01 +/- 0.07. Conclusion. The study demonstrates that TRU, and consequently PCR, can be estimated by on-line measurement of the UV absorption in the spent dialysate.