Diagnosis: Red cell histogram
• The Coulter method is an impedance-based (electrical resistance) method. Cells are suspended in an electrolyte solution and passed through an aperture that has an electric field across it. As cells pass through the aperture, changes in impedance allow the number of cells to be counted and the size of the cell to be determined.
• The number of cells of a particular size that are counted by the analyzer can be shown graphically as a red cell histogram. The number of cells counted for each red cell size is plotted: size on the x-axis and the number on the y-axis.
• The number of red cells can be counted manually by taking a sample, diluting it with an appropriate diluent, and then counting the number of cells in a given volume of the hemocytometer (usually 1 μL). Automated analyzers have replaced this method because it has a high variability and is time and labor intensive.
• The red blood cell count (RBC) on automated hematology analyzers can be determined by determining the area under the curve on the red cell histogram. It is expressed as the number of cells × 1012/L.
• The RBC is not a measure of red cell mass. For example, pregnant women have an increased total blood volume and a lower RBC. However, the total red cell mass is increased.
