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Red Cell Distribution Width (RDW) Test

Reference Range of the Red Cell Distribution Width (RDW)

The red cell distribution width (RDW) test measures variation in red blood cell size or red blood cell volume as a part of a complete blood count (CBC), and it is used along with other RBC indices, especially mean corpuscular volume (MCV), to help determine the causes of anemia.

RDW is elevated in accordance with variation in red cell size (anisocytosis), ie, when elevated RDW is reported on complete blood count, marked anisocytosis (increased variation in red cell size) is expected on peripheral blood smear review.

The reference range for RDW is as follows:

RDW-SD 39-46 fL

RDW-CV 11.6-14.6% in adult

Reference ranges may vary depending on the individual laboratory and patient’s age.

Indications/Applications

Red cell distribution width (RDW) laboratory test is a part of a standard complete blood count (CBC), and it is used along with other RBC indices, especially mean corpuscular volume (MCV) to help determine the causes of anemia.

Considerations

Elevated RDW provides a clue for heterogenous red cell size (anisocytosis) and/or the presence of 2 red cell populations, since other RBC indices (MCV, MCH and MCHC) reflect average values and may not adequately reflect RBC changes where mixed RBC populations are present, such as dimorphic RBC populations in sideroblastic anemia or combined iron deficiency anemia (decreased MCV and MCH) and megaloblastic anemia (increased MCV). Peripheral blood smear review can help confirm the above findings in these circumstances.

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