AbstractThe dialyser clearance of a drug is the sum of two components: one diffusive, arising from the concentration gradient across the membrane, and the other convective, arising from the ultrafiltration of plasma water, produced by the increases in hydraulic pressure that the membrane undergoes.To demonstrate the importance of these clearnces during haemodialysis, this study analyses the influence of a drug's molecular weight on them.To this end, an experimental study of dialysisin vitrowas carried out to determine the clearances, in aqueous solution, of five drugs of increasing molecular weights (theophylline, quinidine, tobramycin, digoxin, and vancomycin), using two series of dialysers with the same type of membrane (Cuprophan®), differing in effective surface area and ultrafiltration coefficient.From the data obtained in this study, the importance of quantifying convective clearance during haemodialysis becomes apparent since if it is not taken into account errors of up to 20% and more may be made. This is particularly so if the drug is of high molecular weight and if a high filtration rate is being used