What Does RDW Mean in a CBC?
RDW (Red Cell Distribution Width) is a standard CBC parameter that measures the variation in size of your red blood cells, with values above 14.0% indicating increased heterogeneity (anisocytosis) and serving as a critical diagnostic tool for anemia evaluation and a prognostic marker for multiple systemic conditions. 1
Technical 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 to express as a percentage. 1 The normal reference range is typically 11.0-14.0%, with values ≤14.0% representing the 95th percentile in reference populations. 1
Primary Clinical Applications in Anemia Diagnosis
Differentiating Microcytic Anemias
When evaluating low MCV (microcytic anemia), RDW provides crucial diagnostic distinction:
- Low MCV + High RDW (>14.0%) strongly indicates iron deficiency anemia 1
- Low MCV + Normal RDW (≤14.0%) suggests thalassemia minor 1
This combination is particularly valuable because it can identify iron deficiency when microcytosis and macrocytosis coexist, resulting in a falsely normal MCV. 1
Evaluating Normocytic Anemia
Normocytic anemia with elevated RDW may indicate early iron deficiency, vitamin B12 or folate deficiency, or hemolysis. 1 The elevated RDW often reflects abnormal erythropoiesis and can be the first indicator of deficiency before microcytosis develops. 1
Comprehensive Diagnostic Workup
RDW should never be interpreted in isolation. 1 The European Crohn's and Colitis Organisation recommends that anemia investigation with RDW must include: 2
- Complete blood count with MCV
- Reticulocyte count
- Serum ferritin
- Transferrin saturation
- C-reactive protein (CRP) levels
Iron Deficiency Diagnosis Based on Inflammation Status
The diagnostic criteria for iron deficiency depend critically on inflammatory status: 2
- Without inflammation: Serum ferritin <30 μg/L confirms iron deficiency 2
- With inflammation present: Ferritin up to 100 μg/L may still indicate iron deficiency 2
- Ferritin 30-100 μg/L: Suggests combined iron deficiency and anemia of chronic disease 2
Beyond Anemia: Prognostic Significance
Cardiovascular Disease and Mortality
Elevated RDW is a strong independent predictor of mortality in the general population. 3 For every 1% increment in RDW, all-cause mortality risk increases by 22%. 3 This association persists even in non-anemic individuals without nutritional deficiencies. 3
The mechanism involves decreased red blood cell deformability in patients with RDW >14.0%, which impairs microcirculatory blood flow and tissue oxygen delivery. 4 This threshold effect (RDW >14.0%) is significantly associated with decreased RBC deformability. 4
Inflammatory and Systemic Conditions
Elevated RDW reflects underlying metabolic derangements including: 5
- Oxidative stress and inflammation
- Impaired erythropoiesis
- Abnormal red blood cell survival
- Poor nutritional status
In inflammatory bowel disease specifically, RDW helps distinguish iron deficiency from anemia of chronic disease, where the latter typically shows ferritin >100 μg/L with transferrin saturation <20%. 1
Critical Pitfalls to Avoid
Do not empirically treat with iron based solely on anemia and elevated RDW. 1 Always confirm iron deficiency with ferritin and transferrin saturation, as the interpretation depends on inflammatory status. 2
Do not ignore elevated RDW even with normal hemoglobin. 5 RDW elevation may precede anemia development and carries independent prognostic significance for cardiovascular events and mortality. 3
Laboratory variation exists between instruments, so interpret values according to your specific laboratory's reference range. 1
When to Pursue Further Investigation
If anemia persists despite appropriate treatment or if there is transfusion dependence, consider gastrointestinal investigation or hematology consultation. 1 Additional testing should include vitamin B12, red cell folate, haptoglobin, and lactate dehydrogenase if the cause remains unclear after initial workup. 2