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Full Blood Count
Full Blood Count

Red Cell Distribution Width

February 15th, 2021
On this page:Anisocytosis


Red cell distribution width (RDW) is the amount of variation in red blood cell size. Cells may be relatively uniform in size and have a low RDW, or have a wide range of sizes and have a high RDW.
    • Normal Range

    • 11 - 15%


Elevated red cell distribution width (RDW) indicates that red blood cells are at a large range of sizes. This phenomenon is known as anisocytosis.
In iron deficiency and megaloblastic anaemia, the RDW may elevate before the MCV becomes abnormal, and as such the RDW can be an early marker.
  • Causes of Anisocytosis

  • Predominantly Microcytic

  • Iron deficiency anaemia
  • Sideroblastic anaemia
  • Predominantly Normocytic

  • Early iron deficiency anaemia
  • Early megaloblastic anaemia
  • Red cell fragmentation
  • Immune haemolysis
  • Sickle cell anaemia
  • Sideroblastic anaemia
  • Blood transfusion
  • Predominantly Macrocytic

  • Megaloblastic anaemia (B12 / folate deficiency, drugs)
  • Myelodysplastic syndrome
  • Chronic liver disease
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