Red Cell Distribution Width (RDW)

The RDW is an index of the variation in cell volume within the red cell population. It is a parameter provided by automated hematology analyzers and is the electronic equivalent of anisocytosis or variation in red blood cell size that is judged by smear examination. Mathematically, it is the coefficient of variation, i.e.,

RDW = (Standard deviation ÷ mean cell volume) x 100

A high RDW indicates that the red blood cells are more variable in volume than normal. This may be due to the presence of smaller or larger red blood cells or a combination of either scenario. For example, increased numbers of immature red blood cells during a regenerative response to an anemia will increase the RDW, because immature anucleate red blood cells are larger than normal. Conversely, the presence of increased numbers of smaller cells (e.g. in iron deficiency anemia) will also increase the RDW (see image below). The cause for a high RDW may be revealed by examination of a blood smear to identify the presence of small or large red blood cells. Note that low numbers of smaller or larger red blood cells may increase the RDW BEFORE you see an increase or decrease in the mean cell volume (MCV) result on the hemogram.

An RDW within reference intervals provides little information on variaton in red blood cell size. An RDW below the reference interval is not a clinically relevant finding.

Red blood cell volume histograms from an impedance-based hematology analyzer (left images) and representative pictures (right images) of Wright's-stained peripheral blood smears from a dog with iron deficiency anemia before (top images) and after transfusion (bottom images). 

Before transfusion, there are large numbers of microcytic hypochromic red blood cells with a small degree of polychromasia and fragmentation (acanthocytes, schistocytes and keratocytes) evident in the dog's peripheral blood. The MCV (37 fL) and MCHC (28 g/dL) are markedly decreased and the RDW, which is graphically illustrated as the width of the histogram curve (delineated by the red double arrow), is also increased (24%) compared to a healthy dog. After transfusion, there are two populations of red blood cells in the dog's peripheral blood: the dog's own microcytic hypochromic fragmenting cells and normocytic normochromic transfused red blood cells. This results in two peaks on the red blood cell volume histogram, with the left peak representing the dog's own microcytic red blood cells and the right peak respresenting the transfused cells. The transfusion has naturally increased the MCV and MCHC to 49 fL and 32 g/dL, respectively. The RDW has almost doubled as seen by the much larger width of the combined curves on the histogram.