Hematocrit has been considered as an important marker of oxygen carrying capacity of the blood associated with whole blood viscosity to assess the mortality risk of both cardiovascular, cerebral, and kidney diseases. The present study developed a new on-line conductivity cell to measure the hematocrit of whole blood using a bipolar square-wave voltage signal at a frequency of 5 kHz. By applying such a voltage signal to a blood flowing in a range of 1.2-3.0 mL/min, the electrolyte effect of blood plasma, the conductive and capacitive effects of blood cells, and the sedimentation effect of erythrocytes on the conductivity of whole blood could be minimized for an accurate hematocrit measurement. The coefficient of correlation between the present method and conventional microcentrifuge method showed an excellent linear relationship. A new equation between the specific conductance and Hct of whole blood was obtained: C = 12.561 - 0.1527 Hct (P < 0.001). (C) 2011 Elsevier Ltd. All rights reserved.