What is RDW (Red Cell Distribution Width) in a Complete Blood Count?
RDW is a calculated measure that quantifies the variation in red blood cell size (anisocytosis), expressed as a percentage, and is routinely reported in a standard CBC to help differentiate causes of anemia and assess overall red blood cell heterogeneity. 1
Definition and Calculation
RDW is calculated by dividing the standard deviation of red blood cell volume by the mean corpuscular volume (MCV) and multiplying by 100:
RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} × 100 1
- A high RDW is generally defined as >14.0%, which corresponds to the 95th percentile of the reference population 1
- The specific RDW value obtained depends on the instrument used, so reference ranges may vary slightly between laboratories 1
Clinical Applications in Anemia Diagnosis
Differentiating Iron Deficiency from Thalassemia
The primary diagnostic use of RDW is distinguishing iron-deficiency anemia from thalassemia minor when microcytosis (low MCV) is present: 1
- Low MCV + RDW >14.0% → indicates iron-deficiency anemia 1
- Low MCV + RDW ≤14.0% → indicates thalassemia minor 1
This distinction is critical because iron-deficiency anemia causes greater variation in red blood cell size than thalassemia minor 1
Detecting Combined Deficiencies
An elevated RDW can identify combined nutritional deficiencies (e.g., iron plus folate or B12 deficiency) where microcytosis and macrocytosis coexist and neutralize each other, resulting in a deceptively normal MCV: 1
- In these situations, a high RDW reveals the underlying heterogeneity in red cell populations that would otherwise be missed 1
- This is particularly important in inflammatory bowel disease patients where multiple deficiencies commonly overlap 1
Role in Inflammatory Bowel Disease
In IBD patients, RDW should be included in the minimum anemia workup alongside MCV, reticulocyte count, serum ferritin, transferrin saturation, and CRP: 1
- A wide size range of red cells (high RDW) serves as an indicator of iron deficiency, especially when inflammation complicates the interpretation of other markers 1
- RDW helps distinguish between iron deficiency anemia and anemia of chronic disease when both conditions overlap 1
Beyond Hematology: Emerging Clinical Significance
While RDW was traditionally used only for anemia diagnosis, recent evidence demonstrates broader clinical implications:
- Elevated RDW is associated with increased cardiovascular disease risk and mortality, though the mechanisms remain incompletely understood 2, 3, 4, 5
- Higher RDW values reflect greater red blood cell heterogeneity and may indicate impaired erythropoiesis, oxidative stress, inflammation, or poor nutritional status 5
- RDW has been studied as a prognostic marker in various conditions including heart failure, sepsis, and malignancies 6, 7, 8
Important Caveats
The diagnostic value of RDW is instrument-dependent and lacks complete standardization across laboratories, so clinicians should use their local laboratory's reference ranges 1
RDW should never be interpreted in isolation—it must be combined with MCV, reticulocyte count, serum ferritin, and clinical context to guide appropriate diagnosis and management 1