Changing hemodialysis thresholds for optimal survival

Background. The urea reduction ratio (URR). a measure quantitating solute r emoval during hemodialysis, is the fractional reduction of the blood urea c oncentration during a single hemodialysis treatment. The URR is the princip al measure of hemodialysis dose in the United States. Based on studies of p atients dialyzed prior to 1994. a minimum URR value of 65% was recommended to optimize survival. Because of new hemodialysis technologies and evolving demographics of the hemodialysis population, the relationship between the amount of hemodialysis and mortality was examined in contemporary cohorts. Methods. This retrospective cohort included >15,000 patients per year recei ving hemodialysis during 1994 through 1997. Each patient's URR was averaged for the three months prior to the beginning of each year. Mortality odds r atios were calculated for patients by URR. To determine the URR value above which no further improvement in mortality was seen (''threshold"), spline functions were tested in logistic regression models, both unadjusted and ad justed for case mix measures. The strength of fit for URR, defined by a ran ge of candidate thresholds from 55 to 75%, was evaluated in increments of 1 % for each year using spline functions. Results. The median URR was 63.2, 65.4, 67.4, and 68.1% for 1994 through 19 97, respectively. The median length of hemodialysis treatments increased on ly six minutes from the beginning to the end of the period of analysis. Usi ng spline functions, the threshold URR values were 61.1, 65.0, 68.0. and 71 .0% for 1994 through 1997 in models adjusted for case mix. The ratio of med ian URR to URR threshold decreased from 1.03 in 1994 to 0.97 in 1997. Conclusions. From 1994 to 1997, the median URR and the URR threshold for mo rtality benefit increased. Although an increased need in the amount of hemo dialysis may be a consequence of changes in patients' demographic character istics, the likely explanation(s) is a change in the dialysis procedure and /or blood sampling favoring higher URR values without changing the amount o f dialysis provided. The recommended minimum URR of 65% appears to be too l ow to confer an optimal mortality benefit in the context of current practic es.