Red blood cell distribution width


Red blood cell distribution width is a measure of the range of variation of red blood cell volume that is reported as part of a standard complete blood count. Usually red blood cells are a standard size of about 6–8 μm in diameter. Certain disorders, however, cause a significant variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range of RDW-CV in human red blood cells is 11.5–14.5%. If anemia is observed, RDW test results are often used together with mean corpuscular volume results to determine the possible causes of the anemia. It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause.
Deficiencies of Vitamin B12 or folate produce a macrocytic anemia in which the RDW is elevated in roughly two-thirds of all cases. However, a varied size distribution of red blood cells is a hallmark of iron deficiency anemia, and as such shows an increased RDW in virtually all cases. In the case of both iron and B12 deficiencies, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated. An elevated RDW is known as anisocytosis.
An elevation in the RDW is not characteristic of all anemias. Anemia of chronic disease, hereditary spherocytosis, acute blood loss, aplastic anemia, and certain hereditary hemoglobinopathies may all present with a normal RDW.

Calculations

The "width" in RDW-CV is sometimes thought to be "misleading", since it in fact is a measure of deviation of the volume of RBCs, and not directly the diameter. However, "width" refers to the width of the volume curve, not the width of the cells. Thus, it is a reasonably accurate term.
Mathematically, the RDW-CV is calculated with the following formula:

Pathological implications

Normal RDW

Anemia in the presence of a normal RDW may suggest thalassemia. A low Mentzer Index, calculated from CBC data , may suggest this disorder but a hemoglobin electrophoresis would be diagnostic.Anemia of chronic diseases show normal RDW.

High RDW

High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells.