What is the diagnosis and management for a patient with microcytic anemia and significant red blood cell size variation, with lab values indicating low hemoglobin, hematocrit, and MCV?

Diagnosis: Iron Deficiency Anemia

This patient has iron deficiency anemia (IDA), confirmed by the combination of severe microcytosis (MCV 70.7), elevated RDW-SD (42.3), and low hemoglobin (11.9 g/dL), which together have 91.4% accuracy for IDA. 1, 2

Immediate Diagnostic Workup

Order serum ferritin and transferrin saturation immediately to confirm iron deficiency before initiating treatment. 1

• Serum ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 2
• Ferritin <45 μg/L provides optimal sensitivity and specificity in clinical practice 2
• Transferrin saturation (TSAT) <15-16% supports iron deficiency and is less affected by inflammation than ferritin 1
• Check C-reactive protein (CRP) to interpret ferritin in the context of inflammation, as ferritin up to 100 μg/L may still indicate iron deficiency if inflammation is present 3, 1, 2

Why This Is Iron Deficiency and Not Thalassemia

The markedly elevated RDW-SD (42.3) essentially rules out thalassemia trait, which typically presents with low MCV but normal or only slightly elevated RDW. 2, 4 In thalassemia, the RDW is typically ≤14.0%, whereas this patient's RDW is dramatically elevated, reflecting the heterogeneous red cell population characteristic of progressive iron depletion. 1, 2

Investigation for Underlying Blood Loss

All adult men and postmenopausal women with confirmed iron deficiency require gastrointestinal investigation unless there is obvious non-gastrointestinal blood loss. 1

• Upper endoscopy with small bowel biopsies reveals a cause in 30-50% of patients and is necessary to exclude gastrointestinal malignancy 1
• Colonoscopy should also be performed to evaluate for lower GI sources of bleeding 1
• In premenopausal women, consider menstrual blood loss as the primary cause, but still investigate if losses seem excessive or if oral iron fails 2

Treatment Protocol

Start oral ferrous sulfate 200 mg (65 mg elemental iron) three times daily, which is the standard, inexpensive, and well-tolerated first-line treatment. 1, 2

• Add ascorbic acid (vitamin C) to enhance iron absorption 2
• Continue treatment for at least 3 months after hemoglobin normalizes to replenish iron stores 2
• Hemoglobin should rise ≥1 g/dL within 2 weeks, confirming the diagnosis of iron deficiency 2

When to Use Intravenous Iron

Switch to intravenous iron (iron sucrose or iron gluconate) if: 1, 2

• No response within 2-4 weeks (consider non-compliance or ongoing blood loss first) 2
• Malabsorption is present 2
• Gastrointestinal intolerance to oral iron 1
• Rapid repletion is needed (heart failure, pregnancy) 1

Expect hemoglobin increase of at least 2 g/dL within 4 weeks with intravenous iron. 3, 2

Common Pitfalls to Avoid

• Do not assume microcytosis equals iron deficiency without confirming with ferritin, as 20-30% of microcytic anemia cases have other diagnoses 5
• Do not stop iron supplementation when hemoglobin normalizes—continue for 3 months to replenish stores 2
• Do not overlook ongoing blood loss as the most common cause of treatment failure 2
• Do not use bone marrow evaluation when serum ferritin and transferrin saturation can establish the diagnosis non-invasively 3, 6