Red blood cell distribution width and cardiovascular diseases

The red blood cells (RBCs), also known as erythrocytes, are non-nucleated blood elements exhibiting a typical oval biconcave shape. Although the normal size of RBCs is usually comprised between 80 and 100 fL, a number of physiological (e.g., pregnancy, ageing or physical exercise) and pathological (e.g., iron deficiency anemia, hemolytic anemia, hereditary spherocytosis, congenital hemoglobin disorders such as thalassemia or hemoglobin variants) conditions may impair erythropoiesis and hence promote a higher degree of heterogeneity of RBC volumes (1). This process is characterized by the appearance of smaller (i.e., <60 fL) and larger (up to 120 fL) elements, which is conventionally known as anisocytosis (Figure 1) (1,2). The red blood cell distribution width (RDW) is a rather simple measure of RBC size heterogeneity, which is calculated by dividing the standard deviation (SD) of erythrocyte volumes for the mean corpuscular volume (MCV) (i.e., RDW = SD/MCV). Although result can hence be expressed either in absolute values (i.e., RDW-SD) or as a percentage (i.e., RDW-%), the latter approach is more widely used in routine laboratory practice. Since the RDW is not a direct measure of anisocytosis, but can be easily and inexpensively calculated from the MCV, the vast majority of hematological analyzers automatically provide the RDW value within the complete blood cell count (CBC). It is noteworthy, however, that the different approaches used for measuring the erythrocyte size (i.e., impedance or optical techniques), as well as the size limits and the relative height of the RBC histogram used for the calculation differ widely across commercial hematological analyzers, so that the clinical usefulness of this parameter is still plagued by a poor degree of harmonization among different manufacturers (3). Accordingly, no universal reference range can be used for this measure, since values are typically instrument-dependent and the physiologic range of values may vary between a minimum of 11% and a maximum of 15%, respectively