Red blood cell distribution width
The red blood cell distribution width (RDW or RCDW) is a measure of the variation of red blood cell (RBC) 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. Certain disorders, however, cause a significant variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range in human red blood cells is 11–15%. If anemia is observed, RDW test results are often used together with mean corpuscular volume (MCV) 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. Vitamin B12 deficiency produces a macrocytic anemia (large cell anemia) with a high RDW. However, iron deficiency anemia initially presents with a varied size distribution of red blood cells, and as such shows an increased RDW. In the case of a mixed iron and B12 deficiency, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated. An elevated RDW (red blood cells of unequal sizes) is known as anisocytosis.
The "width" in RDW is sometimes thought of as "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 (distribution width), not the width of the cells. Thus, it is a reasonably accurate term. 
Mathematically the RDW is calculated with the following formula:
Pathological Implications 
Normal RDW 
When anemia is seen in the presence of a normal RDW, one must have a high suspicion towards Thalassemia as the cause of anemia and a Mentzer Index should be done from the CBC report itself to confirm.
High RDW 
Iron Deficiency Anemia: usually presents with high RDW with low MCV
Folate and vitamin B12 deficiency anemia: usually presents with high RDW and high MCV
Mixed Deficiency (Iron + B12 or folate) anemia: usually presents with high RDW with MCV being high, low or often normal range
Recent Hemorrhage: typical presentation is high RDW with normal MCV
See also 
- The red blood cell distribution width.
- Pomeranian Medical University > Red blood cell indices (indexes) Retrieved on 5 April, 2009
- Cornell University, Department of Population Medicine & Diagnostic Sciences > Red Cell Distribution Width (RDW) Last Updated: June 2000. Retrieved on 5 April, 2009
- Encyclopedia of Surgery: A Guide for Patients and Caregivers Red blood cell indices Retrieved on 5 April, 2009
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