Treatment with erythropoietin and loss of dialyser clearance

Hyperkalaemia as well as increased predialysis values for creatinine and phosphate have been reported in many clinical recombinant human erythropoietin (rHu-EPO) trials in haemodialysis patients. These problems were overcome in most cases by increasing the dose of dialysis. With the exception of urea clearance, solute clearances should decrease with increased haematocrit values. Urea in red blood cells is in diffusion equilibrium with plasma water and it can be assumed that urea clearance is only minimally affected by an increasing haematocrit. Equilibration between red cells and plasma for larger solutes (creatinine) is slow compared to dialyser transit time, so that intracellular creatinine is effectively prevented from participating in the dialytic exchange. Therefore, as the haematocrit increases, the amount of creatinine removed should decrease. These considerations are particularly important for red cell potassium and phosphate, which virtually do not equilibrate with plasma water. In fact, loss of clearance has been reported to range from 5% to 10% for urea and 15% to 20% for creatinine, potassium and phosphate after correction of anaemia. This is particularly relevant in the setting of rapid, high efficiency dialysis with treatment times of 2 h, where more frequent episodes of hyperkalaemia and increased requirements of phosphate binders have been reported. For those who prescribe dialysis by using urea modelling (Kt/V), the dialyser clearance K may have to be corrected for effective blood water clearance and the dose prescribed should be increased, because urea modelling will overestimate the clearance of other uraemic solutes.