Red cell distribution width (RDW)

Last updated: Thursday, 25, March, 2004

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Item Process

5 mL blood in EDTA tube.

2 mL blood in special paediatric EDTA tube.


Part of FBC on many automated instruments.

Reference Interval

Varies with instrument - consult laboratory.


The RDW may assist in the classification of anaemia, in association with the blood film, and the other red cell indices (MCV, MCH and MCHC).


Quantitative assessment of the degree of variation in red cell size/volume (anisocytosis).


Brittenham GM and Koepke JA. Arch Pathol Lab Med 1987; 111: 1146-1148.