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Red Cell Distribution Width

Red cell distribution width (RDW) is an automated determination of the distribution of red blood cell size and corresponds to the morphological term, anisocytosis. Hematology analyzers measure red cell size and calculate the RDW based on either the coefficient of variation in, or the standard deviation of, the red cell population.

The most commonly calculated RDW is based on the coefficient of variation of the red blood cell distribution volume. RDW-CV is calculated using the formula:

RDW-CV=1SD x 100/MCV

In adults, the reference range of RDW-CV is typically 11.0 to 15.0%.

A second method of determining RDW is based on the standard deviation of the red cell distribution width. RDW-SD is an actual measurement of the width of the red cell distribution curve and provides an absolute value in femtoliters (fL). RDW-SD more accurately reflects red cell anisocytosis because it is a directly measured and not influenced by the MCV. In adults, the reference range of RDW-SD is 36-47 fL.

Normally, red cells are uniform in size and a single narrow Gaussian-type histogram is produced. In anemia due to nutritional deficiency anemias (iron, folate, or vitamin B12), homozygous hemoglobinopathies and red cell fragmentation RDW is elevated because of the increasing variability in red cell size.

Normal MCV with increased RDW means both microcytes and macrocytes are present. This combination can occur in patients following transfusion or in patients with any of the following disorders: early iron deficiency (developing microcytes) early folate deficiency (developing macrocytes), homozygous hemoglobinopathies, myelofibrosis, sideroblastic anemia, or reticulocytosis. A normal MCV, increased RDW, macrocytosis and target cells can be seen in liver disease, myelodysplatic syndrome (MDS) and combined iron and folate deficiency.


Constantino BT. The red cell histogram and the dimorphic red cell population. LabMedicine 2011;42(5):300-08. 

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