Diagnosis: Anisocytosis and red cell distribution width (RDW) on 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 red cell distribution width (RDW) is a measure of the spread of the red cell histogram. An elevated RDW corresponds to anisocytosis on peripheral blood smear. Anisocytosis describes the observation of a wide range of differing red cell sizes.
• The RDW can be measured in two ways from the red cell histogram. The RDW-SD (standard deviation) is determined by measuring the distance between the upward and downward slopes of the distribution at 20% of the height. The RDW-CV (coefficient of variation) is calculated as RDW – CV = SD/MCV, where SD is the standard deviation of the distribution and MCV is the mean corpuscular volume (MCV).
• Examples of conditions that lead to an elevated RDW include recent hemorrhage, iron deficiency anemia, treated iron deficiency anemia, sideroblastic anemia, reticulocytosis, B12 and folate deficiency, and others.
