Red cell distribution width (RDW)

What it measures

Shows how much red cells vary in size (anisocytosis). It helps separate iron deficiency from thalassemia trait and can reveal early or mixed nutritional anemias.

Specimen and preparation

• Specimen: Whole blood (EDTA)
• Tube: Lavender top

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Iron deficiency (often early)
• B12 or folate deficiency
• Mixed nutritional anemia
• Recent transfusion or hemorrhage
• Hemolysis with reticulocytosis
• Myelodysplasia

Method

Derived from the red cell volume histogram on the analyzer

Good to know

A low RDW has no clinical significance. High RDW with low MCV favors iron deficiency; normal RDW with low MCV favors thalassemia trait.

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