Clearance of CRRT

Calculate the theoretical clearance of a continuous renal replacement therapy (CRRT) system. This calculation applies to pre-filter replacement configurations only, not post-filter replacement or diafiltration methods. These clearances are theoretical and should be confirmed by other means. Medication dosing should not be based solely on the clearance calculated here.

The units for the Blood Flow Rate, Ultrafiltration Rate, and Replacement Rate settings are usually different from one another. They must first be converted to the same units as the Clearance.

The Plasma Concentration can be for any solute, but the actual value is unimportant; only the ratio to the Ultrafiltrate Concentration matters. (Note that protein binding will diminish actual clearance of specific solutes). Similarly, the units for Plasma Concentration and Ultrafiltrate Concentration cancel out and thus need not be specified. The Ultrafiltrate Concentration equals the Plasma Concentration times the Blood Flow Rate divided by the sum of the Blood Flow Rate and the Replacement Rate (after converting the units). This Ultrafiltrate Concentration would equal the concentration of that solute in the blood returning to the patient if there is no protein binding. If the protein binding is not insignificant, the Ultrafiltrate Concentration would be multiplied by quantity 1 - Protein Bound Fraction.

The formula for the calculation of clearance (in this case creatinine) is:

CrCl =	UCr * UVol
	PCr * Tmin

For continuous renal replacement therapy, the ultrafiltrate is the "urine."

Note that when the Replacement Rate is zero, the Clearance is equivalent to the Ultrafiltration Rate (in mL/min rather than mL/hr). Also note that the initial Plasma Concentration ultimately does not affect the Clearance.

Created: November 4, 2009
Revised: November 5, 2009