Background. Dialysis dose is commonly defined as a clearance scaled to some measure of body size, but the toxicity of uraemic solutes is probably associated more to their concentrations than to their clearance. Methods. 619 dialysis sessions of 35 patients were modified by computer simulations targeting a constant urea clearance or a constant urea concentration. Results. Urea generation rate varied widely in dialysis patients, rather independently of body size. Dialysing to 1.2 in an unselected patient population resulted in great variations in time-averaged concentration (TAC) and average predialysis concentration (PAC) of urea (5.9–40.2 and 8.6–55.8？mmol/L, resp.). Dialysing to equal clearance targets scaled to urea distribution volume resulted in higher concentrations in women. Dialysing to the mean HEMO-equivalent TAC or PAC (17.7 and 25.4？mmol/L) required extremely short or long treatment times in about half of the sessions. Conclusions. The relation between and varies greatly and seems to be different in women and men. Dialysing to a constant urea concentration may result in unexpected concentrations of other uraemic toxins and is not recommended, but high concentrations may justify increasing the dose despite adequate , std？EKR, or std？ . 1. Introduction The morbidity caused by uraemic toxins is probably associated more to their concentrations than to their clearances. Urea is a marker of dialysable uraemic toxins, which, however, are not produced and eliminated in a stoichiometric proportion to urea [1]. Big patients are supposed to need more dialysis than small: higher clearance ( ) and/or more time ( ). Equation where is urea distribution volume, ln natural logarithm, predialysis, and postdialysis concentration, describes the simplest urea kinetic model. It seems just what we want: a measure of dialysis dose automatically scaled to body size (represented by ) and calculated from only two simple variables. It is the basis of the popular second generation Daugirdas equation, which includes empiric correction factors for ultrafiltration and urea generation [2, 3]. The classic single-pool variable-volume urea kinetic model [4, 5] allows to take into account both ultrafiltration and urea generation individually. It requires iterative calculation and correct value of actual dialyser clearance ( ) to give correct urea generation rate and distribution volume . The double-pool model is even more accurate. is a kinetic parameter required in modelling and simulating dialysis. But is -scaled clearance the best variable to correlate dialysis dose to
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