Question 14

With respect to continuous renal replacement therapy (CRRT) in the ICU,

a)  define the terms diffusion and convection and the role they play in solute transport during CRRT

b)  define the terms filtration fraction and sieving coefficient and their significance

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College Answer

a) Diffusion: is the movement of solutes from one compartment to another along a concentration gradient. Diffusion is the principal mode of solute clearance during dialysis. Convection is the movement of solute across a semipermeable membrane in conjunction with significant amounts of ultrafiltration of water (solvent drag). Convection is the principal mode of solute clearance during ultrafiltration.

b) Filtration fraction is the fraction of plasma that is removed from blood during hemofiltration. The optimal filtration fraction at a hematocrit of 30% is of the order of 20-25%. A higher filtration fraction can lead to hemoconcentration in the filter increasing the risk of filter clotting. The sieving coefficient is the ratio of the concentration of solutes in the ultrafiltrate to that of plasma. A high sieving coefficient is desirable for middle molecules but undesirable for albumin sized molecules.


The nomenclature of CRRT is discussed in greater detail in the Required Reading section, as well as in this excellent LITFL summary.

Diffusion: the transport of solute across a membrane, along a concentration gradient. The correct definition is "the net movement of a substance from a region of high concentration to a region of low concentration". This is how small molecules are cleared during haemodialysis.

Convection: the transport of a solute across a membrane along with solvent (by "solvent drag"). The correct definition is "the collective movement of molecules within fluids".  

This is how small and middle molecules are cleared during haemofiltration

Filtration fraction is the volume of plasma removed from the dialysed blood by ultrafiltration. The correct phrase is "the ratio of ultrafiltration rate to plasma water flow rate".

According to the college answer, the ideal filtration fraction at a hematocrit of 30% is 20-25%.

Sieving coefficient is the ratio of a solute in the ultrafiltrate in comparison to its concentration in the plasma which returns to the patient. The correct phrase is "a measure of equilibration between the concentrations of two mass transfer streams". The sieving coefficient describes the efficiency of solute removal by ultrafiltration, and depends upon the properties of the membrane and on the rate of ultrafiltration.


O'Reilly, Philip, and Ashita Tolwani. "Renal Replacement Therapy III: IHD, CRRT, SLED." Critical care clinics 21.2 (2005): 367-378.