A 38-year-old woman presents with fatigue and dyspnea on exertion. Laboratory investigations show hemoglobin 8.5 g/dL, MCV 72 fL, serum iron 45 µg/dL, TIBC 420 µg/dL, ferritin 12 ng/mL, and reticulocyte count 0.8%. Which of the following is NOT consistent with iron deficiency anemia?

Explanation

## Iron Deficiency Anemia — Laboratory Profile ### Key Laboratory Findings in IDA **Key Point:** Iron deficiency anemia presents with a characteristic pattern of iron metabolism derangement. The reticulocyte response is typically blunted because iron is required for hemoglobin synthesis. | Parameter | Iron Deficiency Anemia | Normal Range | |-----------|------------------------|---------------| | Serum Iron | ↓ (< 60 µg/dL) | 60–170 µg/dL | | TIBC | ↑ (> 360 µg/dL) | 250–425 µg/dL | | Ferritin | ↓ (< 30 ng/mL) | 30–300 ng/mL | | Transferrin Saturation | ↓ (< 20%) | 20–50% | | Reticulocyte Count | ↓ or normal (0.5–1.5%) | 0.5–2.5% | ### Why Transferrin Saturation Is ELEVATED in IDA — The Trap **Warning:** This is a classic exam trap. In iron deficiency anemia: - Serum iron is **low** - TIBC is **elevated** (the bone marrow upregulates transferrin receptors to scavenge iron) - Transferrin saturation = (Serum Iron / TIBC) × 100 Calculation in this case: $$\text{Transferrin Saturation} = \frac{45}{420} \times 100 = 10.7\%$$ This is **LOW**, not elevated. Elevated transferrin saturation (> 45%) is seen in **iron overload states** (hemochromatosis, repeated transfusions), not iron deficiency. **High-Yield:** The three correct findings in IDA are: 1. **Low serum iron** ✓ 2. **Elevated TIBC** ✓ (compensatory response) 3. **Low ferritin** ✓ (best marker of iron stores) 4. **Low reticulocyte count** ✓ (iron-limited erythropoiesis) ### Clinical Pearl The blunted reticulocyte response in IDA distinguishes it from acute blood loss (where reticulocyte count rises briskly) and hemolysis (where reticulocyte count is markedly elevated). In IDA, the bone marrow cannot produce adequate hemoglobin despite erythropoietin stimulation because substrate (iron) is limiting. [cite:Harrison 21e Ch 405]