A 35-year-old woman with chronic menorrhagia presents with microcytic hypochromic anemia. Her peripheral blood smear shows target cells and occasional pencil cells. Which single feature best distinguishes iron deficiency anemia from thalassemia trait in this patient?

Question

Accepted Answer

D. Serum ferritin level <15 ng/mL. ## Discriminating Iron Deficiency from Thalassemia Trait ### Key Pathophysiologic Difference **Key Point:** Serum ferritin is the single best discriminator between iron deficiency anemia (IDA) and thalassemia trait because it reflects total body iron stores, which are depleted in IDA but normal or elevated in thalassemia. ### Comparative Features Table | Feature | Iron Deficiency Anemia | Thalassemia Trait | | --- | --- | --- | | **Serum Ferritin** | <15 ng/mL (depleted stores) | Normal or elevated (>100 ng/mL) | | **Serum Iron** | Low (<60 μg/dL) | Normal or high | | **TIBC** | Elevated (>400 μg/dL) | Normal | | **Transferrin Saturation** | <16% | Normal (>20%) | | **Target Cells** | Present in both | Present in both | | **Pencil Cells** | Present in both | Present in both | | **RBC Count** | Low-normal or low | Elevated (>5.5 × 10^6/μL) | | **Mentzer Index (MCV/RBC)** | >13 | <13 | ### Why Ferritin is Superior **High-Yield:** While both conditions present with microcytic hypochromic anemia and target cells, ferritin directly assesses iron stores: - In IDA: iron stores are completely depleted → ferritin <15 ng/mL - In thalassemia trait: iron stores are normal or increased due to chronic hemolysis and transfusions → ferritin normal or elevated **Clinical Pearl:** Ferritin is an acute-phase reactant, so in concurrent inflammation it may be falsely elevated in IDA. In such cases, serum iron and TIBC become more reliable, but ferritin remains the first-line discriminator in uncomplicated cases. ### Why Other Options Fail Target cells, pencil cells, and microcytic hypochromic indices are present in **both** conditions and therefore cannot distinguish between them. The RBC count may help (elevated in thalassemia, low-normal in IDA), but ferritin is more specific and direct. [cite:Robbins 10e Ch 14]

Answer

A. Presence of target cells on blood smear

Answer

B. Microcytic hypochromic RBC indices

Answer

C. Normal or elevated RBC count with low MCV

A 35-year-old woman with chronic menorrhagia presents with microcytic hypochromic anemia. Her peripheral blood smear shows target cells and occasional pencil cells. Which single feature best distinguishes iron deficiency anemia from thalassemia trait in this patient?

Explanation

Discriminating Iron Deficiency from Thalassemia Trait

Key Pathophysiologic Difference

Comparative Features Table

Why Ferritin is Superior

Why Other Options Fail

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Feature	Iron Deficiency Anemia	Thalassemia Trait
Serum Ferritin	<15 ng/mL (depleted stores)	Normal or elevated (>100 ng/mL)
Serum Iron	Low (<60 μg/dL)	Normal or high
TIBC	Elevated (>400 μg/dL)	Normal
Transferrin Saturation	<16%	Normal (>20%)
Target Cells	Present in both	Present in both
Pencil Cells	Present in both	Present in both
RBC Count	Low-normal or low	Elevated (>5.5 × 10^6/μL)
Mentzer Index (MCV/RBC)	>13	<13