Iron Deficiency and Low MCV (Mentzer Index)
Yes, iron deficiency anemia characteristically presents with low MCV (microcytosis), which is a key component of the Mentzer index calculation. Iron deficiency is the most common cause of microcytic anemia and typically produces both low MCV and elevated red cell distribution width (RDW >14.0%) 1, 2.
Understanding the Relationship
Iron deficiency anemia is defined by low MCV (<80 fL in adults) and is the most frequent cause of microcytic anemia worldwide 1, 2. The guideline evidence clearly identifies low MCV as one of the key diagnostic markers for iron deficiency anemia 3.
Key Laboratory Features of Iron Deficiency
- Low MCV is a hallmark finding, typically accompanied by elevated RDW (>14.0%), creating the classic pattern that distinguishes iron deficiency from other microcytic anemias 1, 4
- Other supportive markers include microcytic hypochromic red cells on blood film, low serum iron, raised total iron-binding capacity, and transferrin saturation <16% 3
- Serum ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 3
- In the presence of inflammation, ferritin up to 100 μg/L may still indicate iron deficiency 3
Distinguishing Iron Deficiency from Other Microcytic Anemias
The combination of low MCV with high RDW (>14.0%) strongly suggests iron deficiency anemia, while low MCV with normal RDW (≤14.0%) points toward thalassemia minor 1, 4. This distinction is clinically critical because:
- Iron deficiency: Low MCV + High RDW + Low ferritin 1, 5
- Thalassemia trait: Low MCV + Normal RDW + Normal/elevated ferritin 1, 4, 5
- Anemia of chronic disease: Can cause microcytosis but MCV rarely falls below 70 fL 6
Progressive Changes in Iron Deficiency
The development of microcytosis in iron deficiency follows a predictable pattern 7:
- Early stage: Anisocytosis and increased percentage of microcytic cells appear first, with normal hemoglobin and transferrin saturation <32% 7
- Intermediate stage: MCV and MCH decline, hemoglobin becomes subnormal (but >9 g/dL), transferrin saturation usually <16% 7
- Advanced stage: MCHC becomes low, hemoglobin <9 g/dL, transferrin saturation <16% 7
Critical Clinical Pitfalls to Avoid
- Don't assume all microcytic anemias are iron deficiency—always check ferritin and RDW to differentiate from thalassemia trait 1, 4
- Avoid empiric iron therapy without confirming iron deficiency, as it is ineffective in thalassemia and may cause harmful iron overload 1, 4
- In inflammatory conditions, ferritin may be falsely elevated—consider iron deficiency even with ferritin up to 45-100 μg/L depending on inflammation severity 3, 1
- Combined disorders can coexist—patients with thalassemia trait can develop concurrent iron deficiency, complicating the diagnostic picture 1, 4
Diagnostic Approach
When encountering low MCV 1:
- Check RDW: High RDW suggests iron deficiency; normal RDW suggests thalassemia 1, 4
- Measure serum ferritin: <15 μg/L definitively confirms iron deficiency; <30 μg/L without inflammation; up to 100 μg/L with inflammation 3, 1
- Assess for inflammation: Check CRP to interpret ferritin appropriately 8
- Identify underlying cause: In adult men and post-menopausal women with confirmed iron deficiency, investigate for gastrointestinal blood loss including endoscopy 1