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 corresponding to red cell 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 mean corpuscular volume (MCV) is the average size of the red cells.
• If an automated instrument is not used, then a manual red blood cell count (hemocytometer) and manual hematocrit (packed cell volume) can be used to calculate the MCV. MCV = HCT × 1000/RBC, where HCT is the hematocrit and RBC is the red blood cell count.
• In automated hematology analyzers, the MCV is determined from the mean of the red cell histogram.
• An elevated MCV corresponds to macrocytosis, and a decreased MCV indicates microcytosis, which both can be seen on the peripheral blood smear.
