The aim of this study was to compare concentration measurements of uric acid (UA) removed during dialysis by two algorithms based on UV-absorbance and the 1st derivate of UV absorbance. Ten uremic patients from Tallinn and ten from Linköping, during 30+40 haemodialysis treatments, were followed at the Departments of Dialysis and Nephrology at North-Estonian Medical Centre and at Linköping University Hospital. The dialysate samples were taken and analyzed by means of UA concentration at the chemical laboratory and with a double-beam spectrophotometer. UV absorbance and derivate of UV absorbance was transformed into UA concentration in the spent dialysate using the regression models from the calibration set of material, noted as UV-absorbance (UV_A) and the 1st derivate of UV absorbance (UV_D) method. These models were tested on validation set of material and concentrations of UA from the two methods were compared regarding mean values and SD. Mean concentration of UA were 52.7 +/- 25.0 micromol/l measured at the chemical laboratory (UA_Lab), 54.9 +/- 23.8 micromol/l determined by UV_A and 52.9 +/- 23.0 micromol/l determined by UV_D. The results of mean concentrations were not significantly different (p > or = 0.54). The systematic errors were -7.8 % and -3.3% and random errors were 15.8 % and 10.4 % using UV_A and UV_D respectively. The systematic and random errors were significantly different (p < 0.05) indicating that the new algorithm enables more accurate UA estimation.