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Calculating the Blood Water Urea Clearance During Hemodialysis

Description

Since urea is distributed only in water, use of whole blood flow to estimate urea clearance during hemodialysis results in overestimated values, typically by about 12%. Calculation of the water content of the whole blood allows for more accurate estimates of urea clearance.


 

Physiologic Data:

(1) plasma is about 93% water (varies with protein concentration)

(2) erythrocytes are about 72% water

 

plasma flow rate in mL/min =

= (whole blood flow rate in mL/min) * (1 - (hematocrit as fraction))

 

plasma water flow rate in mL/min =

= (plasma flow rate) * 0.93

 

erythrocyte flow rate in mL/min =

= (whole blood flow rate in mL/min) * (hematocrit as fraction)

 

erythrocyte water flow rate in mL/min =

= (erythrocyte flow rate) * 0.72

 

blood water flow rate in mL/min =

= (plasma water flow rate) + (erythrocyte water flow rate)

 


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