In iron deficiency anemia, which of the following laboratory findings appears FIRST as iron stores become depleted?

Explanation

## Stages of Iron Deficiency **Key Point:** Iron deficiency develops in three sequential stages, and serum ferritin is the earliest marker of depleted iron stores. ### Three Stages of Iron Deficiency Development | Stage | Serum Ferritin | TIBC | Serum Iron | Hemoglobin | RBC Indices | Clinical Significance | |-------|----------------|------|------------|------------|-------------|------------------------| | **Stage 1: Iron Depletion** | ↓ (< 12 ng/mL) | Normal | Normal | Normal | Normal | Iron stores exhausted; no functional iron deficit yet | | **Stage 2: Iron-Deficient Erythropoiesis** | ↓↓ | ↑ | ↓ | Normal | Normal | Functional iron deficit; impaired RBC production | | **Stage 3: Iron Deficiency Anemia** | ↓↓↓ | ↑↑ | ↓↓ | ↓ | Microcytic, hypochromic | Clinical anemia with morphologic changes | **High-Yield:** Serum ferritin is the **most sensitive** marker for detecting iron depletion but is an **acute phase reactant** — it can be falsely elevated in inflammation, malignancy, or liver disease, limiting specificity. ### Chronology of Laboratory Changes 1. **Serum ferritin decreases first** — reflects depletion of iron stores in bone marrow macrophages 2. **TIBC increases** — as iron saturation falls, transferrin synthesis increases to maximize iron-carrying capacity 3. **Serum iron decreases** — functional iron becomes insufficient for erythropoiesis 4. **Hemoglobin falls** — only after functional iron is depleted 5. **Microcytosis develops** — RBCs become smaller as hemoglobin synthesis is impaired **Clinical Pearl:** A patient with normal hemoglobin but low ferritin and elevated TIBC has **iron-deficient erythropoiesis** — iron stores are depleted, but anemia has not yet developed. Early iron supplementation at this stage prevents progression to symptomatic anemia. **Mnemonic: FIST** — **F**erritin (first), **I**ron (second), **S**aturation (third), **T**ransferrin/TIBC (increases as iron falls). [cite:Park 26e Ch 9]