What is RDW in a CBC?
RDW (Red Cell Distribution Width) is a standard component of the complete blood count that measures the variation in size of circulating 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
- The formula is: RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} x 100 1
- Normal RDW values are generally ≤14.0%, which corresponds to the 95th percentile in reference populations 1, 2
- The typical normal range is 11.0-14.0% 2
- RDW values can vary slightly between different laboratory instruments, so interpretation should use the specific laboratory's reference range 2
Clinical Significance in Anemia Diagnosis
RDW is primarily used alongside MCV to differentiate causes of anemia, particularly distinguishing iron deficiency from thalassemia minor. 1, 2
Diagnostic Algorithm for Microcytic Anemia:
- Low MCV + High RDW (>14.0%) = Iron deficiency anemia 1, 2
- Low MCV + Normal/Low RDW (≤14.0%) = Thalassemia minor 1, 2, 3
Interpretation in Other Anemia Patterns:
- Normal MCV with high RDW may indicate early iron deficiency, vitamin B12/folate deficiency, or hemolysis 2
- High RDW can identify iron deficiency when microcytosis and macrocytosis coexist, resulting in a falsely normal MCV 2
- Normal RDW with normal MCV suggests absence of early iron deficiency or vitamin deficiencies 2
Why RDW Matters Beyond Anemia
A normal RDW indicates uniform red blood cell size (absence of anisocytosis), while elevated RDW reflects heterogeneous red cell populations. 2, 4
Broader Clinical Applications:
- Elevated RDW is associated with increased mortality risk in cardiovascular disease, including acute coronary syndrome, heart failure, and stroke 5, 6
- Higher RDW values correlate with decreased red blood cell deformability, which impairs microcirculation and tissue oxygen delivery 7, 8
- RDW increases occur in inflammatory conditions (including COVID-19), cancer, chronic kidney disease, and liver disease 2, 5, 4
Critical Pitfalls to Avoid
- Never assume iron deficiency based solely on low hemoglobin—low RDW makes iron deficiency less likely, not more likely 3
- Do not empirically treat with iron without confirming iron deficiency with ferritin testing, as this can cause iron overload in thalassemia patients 3
- RDW must always be interpreted alongside other CBC parameters (hemoglobin, MCV, reticulocyte count) and clinical context 2
- Even with normal RDW, if anemia is present, minimum workup should include serum ferritin, transferrin saturation, and CRP 2
When to Order Additional Testing
If RDW is elevated (>14.0%) with anemia, the minimum workup should include: 2
- Complete blood count with MCV and reticulocyte count
- Serum ferritin (<30 μg/L suggests iron deficiency without inflammation; <100 μg/L with inflammation) 2, 3
- Transferrin saturation (<16-20% supports iron deficiency) 2
- C-reactive protein to assess for inflammation 3
- Hemoglobin electrophoresis if thalassemia is suspected based on ethnicity and pattern of low MCV with normal/low RDW 3