ACCURACY OF UREA REMOVAL ESTIMATED BY KINETIC-MODELS

The most accurate method for assessing the dialysis dose delivered dur ing high efficiency/flux hemodialysis has not been established. Most c urrent indices of dialysis dose are based on blood-side urea measureme nts, and thus estimate urea removal. Unfortunately, these methods may lead to inappropriately short dialysis during high flux or high effici ency dialysis, perhaps because of inaccuracies in estimating the amoun t of urea removal. It is unknown whether these clearance-based approac hes can accurately predict either absolute or fractional net urea remo val, the latter being equivalent to the solute removal index (SRI). Th erefore, we compared the urea removal calculated by five blood-side ki netic methods: (1) urea reduction ration, (2) 1-pool, (3) 2-pool model s, and the (4) Smye and (5) Daugirdas formulae. These were compared wi th the gold standard measurement by direct dialysate quantification. E ight stable patients receiving high-flux hemodialysis were studied ove r four sessions each. BUN was measured at 0, 45 minutes, 90 minutes, e nd dialysis, one hour after dialysis (equilibrium value), and 48 hours later. Total body water was determined from the dialysate urea remova l; the urea generation rate was calculated using one hour post-dialysi s and 48-hour BUN values. Both the total body water and urea generatio n rate were provided to the 1- and 2-pool models to optimize accuracy. The urea reduction ratio overestimated SRI. The 1-pool model overesti mated both absolute urea removal and SRI in 28 of 32 sessions. The 2-p ool model slightly underestimated both absolute urea removal and SRI. In contrast, the Smye and Daugirdas formulas accurately estimated SRI. We conclude that: (1) The 1-pool model consistently overestimates ure a removal, which leads to inappropriately short dialysis times. (2) Th e 2-pool model, provided with an accurate TBW, slightly underestimates urea removal. (3) The Smye and Daugirdas methods accurately predict S RI and are sufficiently accurate to quantify dialysis dose and adequac y. Because the Smye and Daugirdas methods are operationally and mathem atically simpler than 1- or 2-pool kinetic modeling, we propose that t hey be tested in a randomized controlled trial of dialysis adequacy du ring high efficiency or high flux hemodialysis.