Formula used:

RDW = [Stand Dev of MCV × 100] / MCV

Red Cell Distribution Width is statistical description of heterogeneity of red cell size.

Increased RDW value reflects greater heterogeneity in MCV (anisocytosis)

RDW increases in reticulocytosis, iron deficiency anemia and newborns.

Anemia with normal RDW suspect thalassemia minor.

Normal =11.5-14.5%

Reference:

Anemia: Clinical Approach. In Schmaier AH, Lazarus HM. Concise Guide to Hematology . Wiley-Blackwell 2012.