ABSTRACT
The Coulter principle was the first major advance in automating blood cell counts. The cells to be counted are drawn through a small aperture between two fluid compartments, and the electric impedance between the two compartments is monitored (see Figure 70.1). As cells pass through the aperture, the impedance increases in proportion to the volume of the cell, allowing large numbers of cells to be counted and sized rapidly. Red cells are counted by pulling diluted blood through the aperture. Since red cells greatly outnumber white cells, the contribution of white cells to the red cell count is usually neglected. White cells are counted by first destroying the red cells and using a more concentrated sample.
