What is Red Cell Distribution Width (RDW)?
RDW is a laboratory parameter that measures the variation in size of red blood cells, calculated by dividing the standard deviation of red blood cell volume by the mean corpuscular volume (MCV) and multiplying by 100 to express as a percentage. 1
Technical Definition and Calculation
- RDW is calculated using the formula: RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} x 100 1
- This measurement quantifies anisocytosis (heterogeneity in red blood cell size) and is reported automatically by hematology analyzers 1, 2
- Normal RDW values are generally ≤14.0%, representing the 95th percentile in reference populations 1
- Values typically range from 11.0-14.0% in healthy individuals 1
Clinical Significance of RDW Values
Normal RDW (≤14.0%)
- Indicates a homogeneous red cell population without significant size variation 1
- When combined with low MCV, suggests thalassemia minor rather than iron deficiency anemia 1
- Does not rule out early iron deficiency or vitamin deficiencies, particularly when other indices are abnormal 1
Elevated RDW (>14.0%)
- Strongly indicates iron deficiency anemia when combined with low MCV, as both older normal-sized cells and newer microcytic cells circulate simultaneously 1, 3
- Reflects heterogeneous red blood cell populations due to impaired erythropoiesis and abnormal red cell survival 2
- Can indicate early nutritional deficiencies (vitamin B12, folate) even when MCV remains normal 1
- Occurs in inflammatory conditions including COVID-19 due to erythrocyte membrane injury and reduced deformability 1
Diagnostic Algorithm Using RDW
For Microcytic Anemia (Low MCV)
- Low MCV + High RDW (>14.0%) = Iron deficiency anemia 1, 3
- Low MCV + Normal RDW (≤14.0%) = Thalassemia minor 1, 4
- Confirm iron deficiency with serum ferritin <30 μg/L (or <100 μg/L if inflammation present) and transferrin saturation <16-20% 1, 3
For Normal MCV with Elevated RDW
- High RDW can unmask iron deficiency when microcytosis and macrocytosis coexist, resulting in falsely normal MCV 1
- May indicate early iron deficiency, vitamin B12/folate deficiency, or hemolysis 1
- Requires workup including reticulocyte count, serum ferritin, transferrin saturation, and CRP 1
Essential Workup for Abnormal RDW
- Complete blood count with MCV, hemoglobin, and reticulocyte count 1, 4
- Serum ferritin and transferrin saturation to assess iron status 1, 3
- C-reactive protein (CRP) to identify inflammation that may affect ferritin interpretation 1, 3
- Peripheral blood smear for red cell morphology assessment 4
- RDW must always be interpreted alongside other hematologic parameters, never in isolation 1
Beyond Anemia: Prognostic Value
- Elevated RDW serves as an independent risk factor for mortality in cardiovascular disease, heart failure, stroke, pulmonary embolism, and venous thromboembolism 2, 5, 6
- Increased RDW reflects underlying metabolic derangements including oxidative stress, inflammation, poor nutritional status, and impaired erythropoietin function 2, 6
- The parameter has high negative predictive value for multiple disorders and provides short- and long-term prognostic information 2, 7
Common Pitfalls to Avoid
- Do not empirically treat with iron based solely on anemia without confirming iron deficiency, as low RDW makes iron deficiency less likely 4
- Laboratory reference ranges may vary between instruments; interpret according to your specific laboratory's standards 1
- Ferritin interpretation requires consideration of inflammatory status—use <30 μg/L cutoff without inflammation, <100 μg/L with inflammation 1, 3
- Even with normal RDW, investigate further if clinical suspicion for deficiency exists or other hematologic abnormalities are present 1