Persistent Elevated RDW in a 15-Year-Old Boy
An RDW of 15.5% in a 15-year-old boy indicates anisocytosis (variation in red blood cell size) and most commonly suggests iron deficiency anemia, though the differential diagnosis depends critically on the MCV and clinical context. 1
Diagnostic Approach Based on MCV
The interpretation of elevated RDW (>14.0%) requires immediate assessment of the mean corpuscular volume (MCV) to narrow the differential diagnosis 1:
If MCV is Low (Microcytic)
- Iron deficiency anemia is the most likely diagnosis when RDW >14.0% is combined with low MCV 1
- This combination (low MCV + high RDW) specifically indicates iron deficiency rather than thalassemia minor, which typically presents with low MCV but normal or only mildly elevated RDW (≤14.0%) 1, 2
- Confirm with serum ferritin <30 μg/L (or <100 μg/L if inflammation present) and transferrin saturation <16-20% 2
- In adolescent boys, consider gastrointestinal blood loss, dietary insufficiency, or rapid growth as underlying causes 2
If MCV is Normal (Normocytic)
- Early iron deficiency is a primary consideration, as RDW rises before MCV drops 2, 3
- Hemolysis should be evaluated with reticulocyte count, which will be elevated 4
- Early vitamin B12 or folate deficiency may present this way 2
- Chronic inflammation from conditions like inflammatory bowel disease can elevate RDW even with normal MCV 2, 5
If MCV is High (Macrocytic)
- Vitamin B12 or folate deficiency with active reticulocytosis 2
- Mixed deficiency states (iron deficiency masked by concurrent B12/folate deficiency) 2
Essential Workup
Order these tests immediately to establish the diagnosis: 2
- Complete blood count with MCV
- Reticulocyte count
- Serum ferritin
- Transferrin saturation
- C-reactive protein (CRP) to assess for inflammation
Physiologic Basis
The elevated RDW reflects active erythropoiesis with release of reticulocytes and immature red cells of varying sizes into circulation 4. In iron deficiency specifically, progressive depletion of iron stores leads to increasingly variable red cell production, with newer cells being more microcytic than older cells, creating the characteristic anisocytosis 3, 6.
Common Clinical Scenarios in Adolescent Boys
Iron deficiency anemia (most common):
- RDW typically 15-20% in established deficiency 3, 6
- Sensitivity of 77% for diagnosing iron deficiency anemia when RDW >14.0% 3
- Adolescent growth spurts increase iron requirements 1
Hemoglobinopathies (if family history or ethnicity suggests):
- Sickle cell trait or disease shows elevated RDW (mean 15-20%) 6, 4
- Beta-thalassemia trait shows mildly elevated RDW (mean 15.4%) but less than iron deficiency 6
Chronic disease or inflammation:
- Inflammatory conditions cause erythrocyte membrane injury and reduced deformability 2, 5
- Consider if CRP is elevated 2
Critical Pitfalls to Avoid
- Do not assume all elevated RDW is iron deficiency—always check MCV first to guide the differential 1
- Do not start empiric iron therapy without confirming iron deficiency, as it is ineffective in thalassemia and may cause iron overload 7
- Do not overlook combined disorders—a patient with thalassemia trait can simultaneously develop iron deficiency 7
- Remember that RDW values vary between laboratory instruments, so interpret according to your specific laboratory's reference range 1, 7
When to Suspect Non-Hematologic Causes
While less common in a 15-year-old, persistently elevated RDW has been associated with systemic conditions including chronic kidney disease, liver disease, cardiovascular disease, and diabetes in adults 5. However, in an adolescent, focus first on nutritional deficiencies and hemoglobinopathies before considering these broader systemic causes 2, 6.