What do the red cell distribution width coefficient of variation (RDW‑CV) and red cell distribution width standard deviation (RDW‑SD) labs identify?

What RDW-CV and RDW-SD Labs Identify

RDW-CV and RDW-SD measure the variation in red blood cell size (anisocytosis) and are primarily used to distinguish between different causes of microcytic anemia—specifically to differentiate iron-deficiency anemia from thalassemia minor. 1

Core Function and Calculation

Both parameters quantify red blood cell size variability, but they use different methods:

• RDW-CV (Coefficient of Variation): Calculated as (SD of red blood cell volume / MCV) × 100, expressed as a percentage. Normal is ≤14.0% 1
• RDW-SD (Standard Deviation): The absolute standard deviation of red blood cell volume, measured in femtoliters (fL) 2

RDW-SD appears to be the superior marker in several clinical contexts, showing better discrimination than RDW-CV for iron deficiency anemia 2, 3.

Primary Clinical Application: Differentiating Microcytic Anemias

When you encounter a low MCV (microcytic anemia), RDW helps determine the cause:

• Low MCV + RDW >14.0% → Iron-deficiency anemia 1
• Low MCV + RDW ≤14.0% → Thalassemia minor 1

This distinction is critical because iron-deficiency anemia requires investigation for gastrointestinal blood loss and iron replacement, while thalassemia minor requires genetic counseling and avoidance of unnecessary iron therapy.

Why RDW Increases in Iron Deficiency

Iron-deficiency anemia causes greater variation in red blood cell size than thalassemia minor 1. As iron stores deplete progressively, newly formed red cells become increasingly microcytic while older cells remain normal-sized, creating a mixed population with high variability.

Additional Clinical Context

RDW can identify combined deficiencies: When microcytosis and macrocytosis coexist (e.g., combined iron and folate deficiency), they may neutralize each other, resulting in normal MCV. A high RDW in this setting indicates the underlying size variation and suggests iron deficiency 4.

Minimum workup for anemia should include RDW along with MCV, reticulocyte count, serum ferritin, transferrin saturation, and CRP 4.

Important Caveats

• RDW values depend on the instrument used 1
• RDW-SD has shown better diagnostic performance than RDW-CV for iron-deficiency anemia in mid-pregnancy women (AUC 0.84 vs 0.58) 2
• In inflammatory bowel disease patients, RDW helps distinguish iron-deficiency anemia from anemia of chronic disease when combined with ferritin and transferrin saturation 4, 5