Logarithmic extrapolation of a 15-minute postdialysis BUN to predict equilibrated BUN and calculate double-pool Kt/V in the pediatric hemodialysis population

Blood urea nitrogen (BUN) concentration rebounds logarithmically for 1 hour after a hemodialysis treatment. We have previously devised and evaluated a n equilibrated Kt/V (eqKt/V) estimation method using logarithmic extrapolat ion of the BUN increase from 30 seconds to 15 minutes postdialysis in six p ediatric hemodialysis patients. The current study evaluates logarithmic ext rapolation in 15 additional pediatric patients. Mean measured equilibrated BUN (eqBUN) and estimated BUN at equilibrium (estBUN) using logarithmic ext rapolation were 23.1 +/- 9.2 and 23.0 +/- 9.4 mg/dL, respectively. The mean absolute difference between estBUN and eqBUN was 0.7 +/- 0.4 mg/dL (range, 0.1 to 1.55 mg/dL). All treatments had an absolute difference less than th e SD of the laboratory measurement itself. The mean absolute percentage of difference between eqKt/V using eqBUN and estimated double-pool equilibrate d Kt/V (estKt/V) using estBUN from logarithmic extrapolation was 3.4% +/- 2 .3% and did not vary as a function of patient size, urea generation rate, d ialyzer urea clearance, Kd/V, or ultrafiltration fraction. Mean absolute pe rcentages of difference between eqKt/V and Kt/V estimated by the Tattersall , Daugirdas, or Maduell formulas were 4.5% +/- 3.9%, 4.4% +/- 3.7%, and 6.7 % +/- 8.3%, respectively. Total percentages of error (absolute mean percent age of error + 2 SD) between eqKt/V and estKt/V by logarithmic extrapolatio n or the Tattersall, Daugirdas, or Maduell formulas were 8.0%, 12.3%, 11.8% , and 22.3%, respectively. The greater accuracy of logarithmic extrapolatio n compared with other methods of double-pool Kt/V estimation held true for patients weighing less than 35 kg. We have validated the use of an easy and accurate method requiring only an additional 15-minute posttreatment BUN l evel to estimate double-pool Kt/V in children. (C) 2000 by the National Kid ney Foundation, Inc.