Variation in blood sample collection for determination of hemodialysis adequacy

Inadequate dialysis has been associated with high morbidity and mortality in end-stage renal disease (ESRD) patients receiving maintenance hemodialysis. The accurate estimation of dialysis adequacy, measured either as a calculated urea kinetics (Kt/V) or a simple urea reduction ratio (URR) is dependent on the proper collection of blood samples for predialysis and postdialysis blood urea nitrogen (BUN) determination. Because no established protocol exists for blood sampling, we surveyed the study cohort of dialysis centers participating in the National Kidney Foundation Council on Renal Nutrition National Research Question Collaborative Study to determine the comparability of BUN data that were collected to calculate URR to determine adequacy of dialysis. Surveys were completed by 100% of the 202 units participating: 195 in the United States (from 43 states) and seven from Canada, treating approximately 15,000 hemodialysis patients in total. The distribution of the sample by the type of facility mirrored that of 1996 United States Renal Data System (USRDS) Annual Report facilities data. Results showed a 5.0% error in predialysis blood draw and an 8.4% to 41.6% error in the postdialysis counterpart. There was a large variability in the observed postdialysis methods in general. Dilution of predialysis sample with either heparin or saline will falsely underestimate Kt/V and URR. The presence of access-derived, recirculated blood in the postdialysis sample will falsely overestimate Kt/V and URR. Excessive delay in drawing postdialysis sample will reduce Kt/V and URR because of urea rebound. Adoption by all dialysis providers of a uniform blood sample draw procedure will result in a consistency necessary to allow reliable and valid comparison of adequacy of dialysis parameters within and between ESRD patients, units, and clinical trials. (C) 1998 by the National Kidney Foundation, Inc.