Microcytic Anemia: Diagnosis and Treatment
Based on these lab values showing microcytic anemia (MCV 74, hemoglobin 12.5) with an elevated RDW (15.3), the most likely diagnosis is iron deficiency anemia, and first-line treatment is oral ferrous sulfate 200 mg three times daily for at least three months after hemoglobin correction. 1, 2
Diagnostic Confirmation
The elevated RDW (15.3, above 14.0%) combined with low MCV strongly points toward iron deficiency anemia rather than thalassemia minor, which typically presents with RDW ≤14.0%. 1, 2
Obtain serum ferritin immediately as the most specific diagnostic test:
- Ferritin <15 μg/L confirms absent iron stores 1, 2
- Ferritin <30 μg/L indicates low body iron stores 1, 2
- A cut-off of 45 μg/L provides optimal sensitivity and specificity in clinical practice 1, 2
Also measure transferrin saturation (TSAT), which is more sensitive than hemoglobin alone for detecting iron deficiency. 1, 2
Treatment Algorithm
First-Line Therapy
Initiate oral ferrous sulfate 200 mg (containing 65 mg elemental iron) three times daily. 1, 2, 3 Continue for at least three months after anemia correction to replenish iron stores. 1, 2
Alternative oral formulations if ferrous sulfate is not tolerated:
Expected Response
A good response is defined as hemoglobin rise ≥10 g/L (≥1 g/dL) within 2 weeks, which confirms iron deficiency. 1, 2 If using IV iron, expect hemoglobin increase of at least 2 g/dL within 4 weeks. 1
If Oral Iron Fails
Consider intravenous iron if:
- Malabsorption is present 1, 2
- Patient cannot tolerate oral formulations despite trying alternatives 1
- Hemoglobin fails to rise appropriately after 2-4 weeks 1
Monitoring Protocol
Follow hemoglobin and red cell indices:
- Every 3 months for the first year 1, 2
- Once more after an additional year 1, 2
- Provide additional oral iron if hemoglobin or MCV falls below normal 1, 2
Monitor iron stores with serum ferritin and transferrin saturation to assess adequacy of repletion. 1
Critical Pitfall: Rule Out Genetic Disorders
If the patient fails to respond to adequate oral iron therapy despite confirmed compliance, consider genetic disorders of iron metabolism or heme synthesis. 1, 2, 4 This is particularly important given the significantly low MCV of 74, which can suggest:
- IRIDA (iron-refractory iron deficiency anemia) - requires IV iron, though rarely achieves complete normalization 1
- SLC11A2 defects - present with microcytic anemia and increased TSAT 2, 4
- Sideroblastic anemias (ALAS2, SLC25A38, STEAP3, ABCB7 defects) 5, 4
- Hypotransferrinemia - characterized by low transferrin, low serum iron, high ferritin 2, 4
If TSAT is paradoxically elevated with microcytic anemia, genetic testing should be pursued immediately. 2
Investigate Underlying Cause
Iron deficiency anemia is not a diagnosis—it requires identification of the source of iron loss:
- Gastrointestinal blood loss - most common in men and post-menopausal women; may require upper and lower endoscopy 2
- Menstrual blood loss - if patient is female of reproductive age 2
- Poor dietary intake - assess nutritional history 2
- Malabsorption - celiac disease, inflammatory bowel disease 2
- NSAID use - causing occult GI bleeding 2
Special Considerations
Avoid missing combined deficiencies: Iron deficiency can coexist with B12 or folate deficiency, which may mask the typical microcytic picture. 2
Monitor for iron overload if patient requires multiple transfusions or long-term therapy, particularly if genetic disorder is identified. 1, 4 Consider liver MRI in specific cases to detect toxic iron loading early. 1, 4
If ferritin is normal or elevated despite microcytic anemia, do not assume iron deficiency—this suggests anemia of chronic disease, thalassemia, or genetic disorders requiring different management. 2, 4