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The red blood cell distribution width, or RDW, is a measure of the variation of red blood cell width that is reported as part of a standard complete blood count. Usually red blood cells are a standard size of about 6-8μm. 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 MCV results to figure out what the cause of the anemia might be. It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause. The RDW is based on the MCV as it is the MCV that measures the size of the red cell. B12 deficiency produces a macrocytic (large cell) anemia with a normal RDW. However, iron deficiency anemia initially presents with a varied size distribution of red blood cells and shows an increased RDW. And in the case of a mixed iron and B12 deficiency we will have a mix of both large cells and small cells hence the RDW will usually be significantly elevated. An elevated RDW, i.e. red blood cells of unequal sizes, is known as anisocytosis.
Mathematically the RDW is calculated with the following formula:
- RDW = (Standard deviation of red cell width ÷ mean cell width) × 100
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