In iron deficiency anemia, which of the following iron studies findings is EARLIEST to appear?

Explanation

## Stages of Iron Deficiency and Laboratory Progression **Key Point:** Iron deficiency develops in three progressive stages, and serum ferritin is the EARLIEST marker to become abnormal. ### Three Stages of Iron Deficiency | Stage | Iron Stores | Serum Iron | TIBC | Ferritin | Hemoglobin | RBC Morphology | |-------|-------------|-----------|------|----------|------------|----------------| | **Stage 1: Iron depletion** | ↓ | Normal | Normal | ↓ | Normal | Normal | | **Stage 2: Iron-limited erythropoiesis** | ↓ | ↓ | ↑ | ↓ | Normal/↓ | Normocytic → Microcytic | | **Stage 3: Iron deficiency anemia** | ↓ | ↓ | ↑ | ↓ | ↓ | Microcytic hypochromic | **High-Yield:** The sequence of abnormalities is: 1. **Serum ferritin ↓** (earliest — reflects depleted iron stores) 2. **Serum iron ↓ + TIBC ↑** (iron-limited erythropoiesis begins) 3. **Hemoglobin ↓** (anemia manifests) 4. **RBC morphology abnormal** (latest — microcytic hypochromic) **Mnemonic: "FISH"** — **F**erritin first, then **I**ron and TIBC, then **S**erum hemoglobin, then **H**istology (morphology). **Clinical Pearl:** A patient with normal hemoglobin but low ferritin has iron deficiency WITHOUT anemia yet—this is Stage 1 iron depletion, a critical window for intervention before anemia develops.