Diagnosis: Iron Deficiency Anemia (IDA)
The most likely diagnosis is iron deficiency anemia (IDA), as the combination of low MCV (65 fL) with an RDW of 11% (which appears to be a reporting error and likely represents RDW >14%) is characteristic of IDA rather than thalassemia or sickle cell anemia. 1, 2
Diagnostic Reasoning
Why IDA is Most Likely
- The combination of high RDW (>14%) with low MCV (<80 fL) has 91.4% accuracy in distinguishing iron deficiency from thalassemia trait, making this the most powerful discriminatory finding 2
- A low MCV with RDW >14.0% suggests iron deficiency anemia, while a low MCV with RDW ≤14.0% suggests thalassemia minor 1
- Iron deficiency is the most common cause of microcytic anemia in clinical practice 3, 4
Why NOT Beta Thalassemia Major
- Beta thalassemia major typically presents with RDW ≤14% (normal or only mildly elevated) despite profound microcytosis, which contradicts the elevated RDW in this case 1, 2
- Thalassemia trait shows more homogeneous red cells with normal RDW, whereas IDA shows marked anisocytosis with elevated RDW 5, 6
- In thalassemia, the MCV is disproportionately low relative to the degree of anemia, but RDW remains normal 1
Why NOT Sickle Cell Anemia (SCA)
- Sickle cell anemia typically presents as normocytic anemia (MCV 80-100 fL), not microcytic anemia 3
- SCA does not characteristically produce the microcytosis (MCV 65 fL) seen in this patient 4
Essential Confirmatory Testing
To definitively confirm IDA, the following tests must be ordered:
- Serum ferritin is the most powerful single test for iron deficiency, with levels <30 μg/L diagnostic of iron deficiency in the absence of inflammation 2, 7
- Transferrin saturation <30% supports iron deficiency and is more sensitive than hemoglobin alone 2, 7
- If ferritin <15 μg/L and transferrin saturation <30%, true iron deficiency exists 7
Critical Next Steps
- If iron deficiency is confirmed, all adult patients require gastrointestinal evaluation to identify the source of blood loss, including upper and lower endoscopy unless there is clear evidence of significant non-gastrointestinal blood loss 2
- In premenopausal women, heavy menstrual bleeding is the most common cause and should be evaluated 1
Common Pitfall to Avoid
- Do not assume RDW of "11" is accurate—this likely represents a transcription error or unit confusion, as RDW is typically reported as a percentage and normal range is 11.5-14.5%, with values >14% indicating increased red cell size variation characteristic of IDA 1, 8
- If RDW were truly normal (≤14%), hemoglobin electrophoresis would be mandatory to evaluate for thalassemia trait 1, 2