Based on permittivity changes, a new method to measure hematocrit (HCT) in extracorporeal blood systems is presented. Human blood samples were tested at different HCT levels pairing the values of permittivity change, obtained by means of a commercial impedance analyzer, with traditional centrifugation measurements. Data were correlated using both linear and nonlinear regression. When using the lineal model, the comparison yielded a high correlation coefficient (r = 0.99). Theoretical simplifications suggest that the method is independent of changes in the conductivities of the intracellular and extracellular compartments. The influence of osmolarity and conductivity of the extracellular compartment was analyzed. It is shown that HCT can be predicted within an error lower than 5% when those parameters changed as much as 1 mS/cm and 50 mOsm/kg, respectively. Thus, the method appears as valid and viable showing good possibilities in applications such as renal dialysis.

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