Red blood cell distribution width (RDW), as well as various types thereof (RDW-CV or RCDW and RDW-SD), is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count. Red blood cells have an average volume of 80–100 femtoliters, but individual cell volumes vary even in healthy blood.
An elevation in the RDW is not characteristic of all anemias. Anemia of chronic disease, hereditary spherocytosis, acute blood loss, aplastic anemia (anemia resulting from an inability of the bone marrow to produce red blood cells), and certain hereditary hemoglobinopathies (including some cases of thalassemia minor) may all present with a normal RDW.
Calculations
The "width" in RDW 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. RDW-CV "width" refers to the width of the volume curve (distribution width), not the width of the cells.
RDW-SD is calculated as the width (in fL) of the RBC size distribution histogram at the 20% height level. This parameter is, therefore, not influenced by the average RBC size (mean corpuscular volume, MCV).
Since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).
- Iron-deficiency anemia usually presents with high RDW and low MCV.
- Folate and vitamin B<sub>12</sub> deficiency anemia usually presents with high RDW and high MCV.
- Mixed-deficiency (iron + B<sub>12</sub> or folate) anemia usually presents with high RDW and variable MCV.
- Recent hemorrhages typically present with high RDW and normal MCV.
- A false high RDW reading can occur if EDTA anticoagulated blood is used instead of citrated blood. See Pseudothrombocytopenia.
By severity, elevated RDW can be classified as follows:
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! RDW in adults !! Severity