Management of Abnormal Red Cell Distribution Width (RDW)

Understanding RDW Values

RDW is calculated by dividing the standard deviation of red blood cell volume by MCV and multiplying by 100 to express the result as a percentage 1
Normal RDW is generally considered to be less than or equal to 14.0%, which corresponds to the 95th percentile in reference populations 2

When combined with low MCV (microcytosis), an elevated RDW strongly suggests iron deficiency anemia 1, 2
When combined with normal MCV, an elevated RDW may indicate early iron deficiency, vitamin B12 or folate deficiency, or hemolysis 2
Minimum workup should include complete blood count with MCV, reticulocyte count, serum ferritin, transferrin saturation, and CRP 2

The American Gastroenterological Association recommends confirming diagnosis of iron deficiency anemia with serum ferritin (<30 μg/L in absence of inflammation, <100 μg/L with inflammation) 3, 2
Initiate iron supplementation (oral or intravenous) for iron deficiency anemia 2
Investigate potential sources of blood loss, particularly gastrointestinal causes, for iron deficiency anemia 4, 5
For adult men and post-menopausal women with iron deficiency anemia, both upper and lower gastrointestinal endoscopy should be performed to exclude malignancy 4

Assuming iron deficiency based solely on microcytosis without checking RDW 4, 2
Failing to investigate for gastrointestinal malignancy in adult men and post-menopausal women with iron deficiency anemia 4
Overlooking the possibility of combined deficiencies (e.g., iron and B12), which may result in a normal MCV despite abnormal RDW 5, 2
Attributing microcytosis to thalassemia in certain ethnic groups without laboratory confirmation 4

Combined iron deficiency with vitamin B12 or folate deficiency produces a normocytic anemia because the macrocytic effect of the vitamin deficiency masks the microcytic effect of iron deficiency. 6

The coexistence of iron deficiency and vitamin B12/folate deficiency is characterized by an elevated red‑cell distribution width (RDW), reflecting a heterogeneous mixture of microcytic and macrocytic red cells. 6

In patients presenting with normocytic anemia and a high RDW, clinicians should simultaneously evaluate iron status (serum ferritin, transferrin saturation) and vitamin B12/folate levels, because combined deficiencies can maintain a normal MCV despite significant nutrient deficits. 6