The removal of plasma water during hemodialysis and ultrafiltration usually leads to a decrease in plasma volume and to a concomitant increase in the concentration of components not removed by that process. At a baseline hematocrit of 35% the relative change of a component measured per unit plasma volume is almost twice as large as the concomitant change in hematocrit or hemoglobin concentration measured per unit blood volume. Thus, to asses whether the change of a plasma component results from the volume change or from other aspects of the intervention, the plasma concentration measured per unit plasma volume has to be divided by the hemoconcentration for the plasma compartment h(p) = H(1) (100 - H(0))/(H(0)[100 - H(1)]), where H is the hematocrit in percent and where indices 0 and 1 refer to the condition before and after intervention, and not by the hemoconcentration for the blood compartment h(H) = H(1)/H(0), as it is frequently done.