Blood urea concentration is artificially low immediately following high-efficiency dialysis of duration T and will rise (‘rebound’) due to the continued diffusion of urea from the intracellular to the extracellular space. This leads to an overestimate of the efficiency of the dialysis given by KT/V (where V is the total distribution volume of urea and K is the urea clearance of the dialyser) if the true equilibrium blood concentration of urea is not used in the calculation of KT/V by the single-pool urea kinetic model (UKM). The measurement of the equilibrium urea concentration entails an additional blood sample 60 min after dialysis, but an estimate may be calculated using a blood sample taken 80 min following the start of dialysis together with an approximate formula derived from the 2-pool model. In 14 patients, an average error in the calculation of KT/V of 35% (range 19-75%) by the single-pool UKM was reduced to 13% (range 1-55%, but 8 measurements to < 7%) using the approximate technique. It is concluded that the approximate technique significantly improves the accuracy of dose estimation in high-efficiency dialysis without inconveniencing the pati