A 38-year-old woman presents with fatigue and dyspnea on exertion. Her hemoglobin is 8.5 g/dL with microcytic, hypochromic indices (MCV 68 fL, MCH 22 pg). Serum iron is low, TIBC is elevated, and ferritin is 12 ng/mL. Which of the following is NOT a typical finding in iron deficiency anemia?

## Iron Deficiency Anemia: Laboratory Findings ### Pathophysiology Iron deficiency triggers upregulation of iron transport and storage mechanisms. The body attempts to maximize iron absorption and utilization through increased transferrin receptor expression on erythroid precursors. ### Serum Transferrin Receptor (sTfR) in IDA **Key Point:** Serum transferrin receptor (sTfR) is **elevated** in iron deficiency anemia, not decreased. sTfR reflects the number of transferrin receptors shed from the surface of erythroid precursors and is a sensitive marker of iron-deficient erythropoiesis. **High-Yield:** sTfR is one of the earliest markers of iron deficiency and remains elevated even when ferritin levels normalize during iron repletion. It is particularly useful in distinguishing IDA from anemia of chronic disease. ### Typical Laboratory Findings in IDA | Finding | Status in IDA | |---------|---------------| | Serum iron | Decreased | | TIBC | Elevated | | Ferritin | Decreased (< 30 ng/mL) | | Serum transferrin receptor (sTfR) | **Elevated** | | Soluble transferrin receptor (sTfR) | **Elevated** | | MCV | Decreased (< 80 fL) | | MCH | Decreased (< 27 pg) | | Peripheral smear | Microcytic, hypochromic, target cells, pencil cells | ### Peripheral Blood Smear Findings **Clinical Pearl:** The classic morphology of IDA includes: - **Microcytic, hypochromic RBCs** — reduced hemoglobin content and cell size - **Target cells** — central pallor with a central dot due to excess membrane relative to hemoglobin - **Pencil cells** (elliptocytes) — elongated RBCs with reduced hemoglobin - **Occasional poikilocytosis** — variation in RBC shape ### Why sTfR is Elevated (Not Decreased) In iron deficiency, erythroid precursors upregulate transferrin receptors to maximize iron uptake from transferrin. These receptors are shed into the serum as soluble transferrin receptor (sTfR). The sTfR level directly correlates with erythroid proliferation and iron demand, making it elevated in IDA. [cite:Harrison 21e Ch 98]