What Does High RDW Mean on a Full Blood Count?
A high RDW (red cell distribution width) on your FBC indicates increased variation in red blood cell sizes (anisocytosis), which most commonly signals iron deficiency anemia, but can also reflect other underlying conditions requiring further investigation.
Understanding RDW as a Diagnostic Tool
RDW measures the heterogeneity of red blood cell volumes and is calculated as a percentage, with values >14.0% generally considered elevated 1. When RDW is elevated, it serves as an important indicator of iron deficiency and helps distinguish between different types of anemia 2.
Primary Clinical Significance
Iron Deficiency Anemia
The most common cause of elevated RDW is iron deficiency anemia 2. The diagnostic approach depends on MCV:
- Low MCV + High RDW (>14%): Strongly indicates iron deficiency anemia 1
- Low MCV + Normal RDW (≤14%): More suggestive of thalassemia minor 1
This distinction is critical because iron deficiency causes greater variation in red cell size than thalassemia 1.
Mixed Deficiency States
High RDW can reveal combined deficiencies that might otherwise be masked 2. When microcytosis and macrocytosis coexist (from simultaneous iron and B12/folate deficiency), they may neutralize each other resulting in normal MCV. The elevated RDW exposes this hidden problem 2.
Systematic Evaluation Approach
When you encounter high RDW, follow this algorithm:
Step 1: Check MCV
- Microcytic (<80 fL): Pursue iron studies, consider thalassemia
- Normocytic (80-100 fL): Check reticulocyte count
- Macrocytic (>100 fL): Evaluate B12/folate levels
Step 2: Assess Iron Status 2, 3
- Without inflammation: Ferritin <30 μg/L confirms iron deficiency
- With inflammation: Ferritin up to 100 μg/L may still indicate iron deficiency
- Check transferrin saturation (<16% suggests iron deficiency)
Step 3: Additional Workup 2
- Reticulocyte count (distinguishes production vs. destruction)
- CRP (identifies inflammation)
- B12 and folate levels
- Consider hemolysis markers if reticulocytes elevated
Beyond Anemia: Prognostic Implications
Recent evidence demonstrates that elevated RDW predicts adverse outcomes far beyond hematological disorders 4, 5, 6, 7. High RDW is associated with:
- Increased all-cause and cardiovascular mortality 4, 7
- Heart failure and atrial fibrillation risk 4, 5
- Acute kidney injury in critically ill patients 8
- Poor nutritional status and treatment response 9
This broader prognostic value reflects RDW as a marker of systemic inflammation, oxidative stress, and metabolic derangement 6, not just red cell abnormalities.
Common Pitfalls to Avoid
Don't assume anemia is present: RDW can be elevated without anemia, though this still warrants investigation 10.
Don't ignore inflammation: Ferritin is an acute phase reactant. In inflammatory states (elevated CRP), higher ferritin cutoffs (up to 100 μg/L) are needed to diagnose iron deficiency 2, 3.
Don't overlook medication effects: Thiopurines (azathioprine, 6-mercaptopurine) cause macrocytosis that can elevate RDW 2.
Don't forget gastrointestinal evaluation: In confirmed iron deficiency without obvious blood loss, investigate for occult GI bleeding, especially in men and postmenopausal women 10.
Practical Clinical Application
RDW is included in routine complete blood counts, making it a cost-effective screening tool 4, 6. When elevated:
- Confirm with iron studies (ferritin, transferrin saturation)
- Assess for inflammation (CRP)
- Evaluate for combined deficiencies if MCV is normal
- Consider broader prognostic implications in patients with cardiovascular or chronic disease
- Search for underlying causes (blood loss, malabsorption, chronic disease)
The strength of RDW lies in its ability to detect iron deficiency even when other parameters appear normal, and its emerging role as a general marker of physiological stress and disease severity 6, 7.