Red Cell Distribution Width (RDW)

The normal reference range for Red Cell Distribution Width (RDW) is RDW is typically about 11.5-14.5%. It measures how much red cell sizes vary (anisocytosis). On its own it means little, but combined with the MCV it sharpens the diagnosis of anaemia.. Reference ranges may vary between laboratories, so always consult your healthcare provider for interpretation.

Assists in diagnosing different types of anemia and has been linked to overall health status and mortality risk.

A high RDW means red cells vary widely in size, which happens when a new population of differently sized cells is being produced. It is often the earliest laboratory sign of iron deficiency, sometimes appearing before the MCV or haemoglobin change. It is also raised by mixed deficiencies (iron plus B12 or folate), recent blood transfusion, and ongoing haemolysis. A high RDW with a low MCV strongly suggests iron deficiency, whereas thalassaemia typically shows a low MCV with a relatively normal RDW.

A low RDW (uniform red cell size) has no clinical significance and requires no action.

RDW is calculated automatically as part of the red cell indices in every full blood count, so it is never ordered alone. It is used together with the MCV to direct anaemia testing — for example, prompting iron studies when both point to iron deficiency.