Microcytic Anemia with Elevated RDW: Iron Deficiency
The most likely diagnosis is iron deficiency anemia, and the next diagnostic step is to measure serum ferritin and transferrin saturation. 1, 2
Key Laboratory Findings
Your CBC demonstrates a classic pattern of iron deficiency:
- Low MCV (78 fL) indicates microcytic red blood cells, which is the hallmark of iron deficiency anemia 1
- Low MCH (23.8 pg) and low MCHC (30.4 g/dL) confirm hypochromic (pale) red blood cells with reduced hemoglobin content 1
- Elevated RDW (16.6%) reflects increased variation in red blood cell size (anisocytosis), which occurs early in iron deficiency as the body produces both normal and iron-deficient cells 1, 3
- Elevated RBC count (5.60) with relatively preserved hemoglobin (13.3 g/dL) suggests compensatory production of smaller cells to maintain oxygen-carrying capacity 1
Immediate Diagnostic Workup
Order these tests immediately to confirm iron deficiency: 2, 4
- Serum ferritin - the most specific test for iron stores; levels <30 μg/L confirm iron deficiency 2
- Transferrin saturation - calculated from serum iron and total iron binding capacity; <20% indicates iron deficiency 1, 2
- C-reactive protein (CRP) - essential because inflammation falsely elevates ferritin; in inflammatory states, ferritin up to 100 μg/L may still indicate iron deficiency 2, 4
Critical Diagnostic Considerations
The combination of low MCV with elevated RDW has high specificity for iron deficiency anemia. 1, 3 The CDC guidelines emphasize that while hemoglobin alone cannot determine the cause of anemia, the pattern of microcytosis (low MCV) with increased RDW strongly suggests iron deficiency rather than thalassemia trait (which typically shows low MCV with normal RDW). 1
Important caveat: If ferritin is normal or elevated but CRP is also elevated, you may have coexisting iron deficiency masked by inflammation. 2, 4 In this scenario, transferrin saturation becomes the more reliable indicator.
Alternative Diagnoses to Consider
While iron deficiency is most likely, also evaluate for:
- Thalassemia trait - typically shows more severe microcytosis (MCV <75 fL) with normal or low-normal RDW and elevated RBC count, but your elevated RDW makes this less likely 1
- Anemia of chronic disease - can cause microcytosis but usually shows normal or elevated ferritin with low transferrin saturation 1
- Mixed deficiency states - the elevated RDW is a critical clue that dual pathology (e.g., coexisting B12/folate deficiency with iron deficiency) may be present 2, 4
When to Refer to Hematology
Obtain hematology consultation if: 2, 4
- Ferritin and transferrin saturation do not confirm iron deficiency
- No response to appropriate iron replacement after 2-3 weeks
- Hemoglobin continues to decline despite treatment
- Any concern for underlying hematologic disorder
Clinical Context Matters
The relatively preserved hemoglobin (13.3 g/dL) despite clear microcytosis suggests early or chronic compensated iron deficiency. 1 This pattern often occurs with slow, ongoing blood loss (menstrual, gastrointestinal) where the body adapts by producing more red blood cells, albeit smaller ones.