The different forms of continuous renal replacement therapy (CRRT) are generally reserved for critically ill patients in whom errors in drug dosing may be particularly deleterious . Thus, knowledge of factors that determine drug removal in this setting is essential .
This discussion is limited to the removal of drugs by CRRTs. Drug elimination is the sum of regional clearances, including metabolism as well as the removal by an extracorporeal therapy, as are most CRRTs (exception is peritoneal dialysis). In the settings where CRRTs may be applied, the metabolic clearances of drugs may be different from the metabolism of that drug in the normal circumstance. That concept is not discussed in this topic review.
Hemofiltration refers to the removal of an ultrafiltrate of plasma in which there is no solute loss by diffusion since the plasma flowing through the filter is not exposed to dialysate flowing on the other side of the membrane (see "Renal replacement therapy (dialysis) in acute kidney injury (acute renal failure): Metabolic and hemodynamic considerations"). Solute removal during hemofiltration occurs by convection (or solvent drag). The frictional forces between water and solutes result in the movement of small and middle molecular weight solutes (<5000 daltons) in the same direction as plasma water.
Sieving coefficient — The efficiency of the removal of drugs or any other solute is related to the sieving coefficient (SC). The SC is the mathematical expression of the ability of a solute to convectively cross a membrane. It is determined from the ratio of the solute concentration in the ultrafiltrate to the solute concentration in the plasma. Since the arterial and venous concentrations may be different because of solute removal, it is more accurate to estimate the SC during hemofiltration from the average of the arterial (A) and venous (V) concentrations:
SC = [UF] ÷ ([A] + [V] ÷ 2)