A 35-year-old man with chronic gastrointestinal blood loss presents with microcytic hypochromic anemia (Hb 7.5 g/dL, MCV 58 fL). Serum ferritin is 12 ng/mL (normal >30). Which is the most appropriate next investigation to confirm iron deficiency and assess iron stores before starting therapy?

Explanation

## Investigation Strategy in Suspected Iron Deficiency Anemia ### Rationale for Serum Transferrin Saturation and TIBC **Key Point:** When serum ferritin is low (<30 ng/mL) in a microcytic anemia, the next step is to measure serum iron and TIBC to calculate transferrin saturation. A saturation <16% confirms iron deficiency and quantifies the degree of iron depletion. **High-Yield:** The diagnostic algorithm for iron deficiency anemia: 1. Microcytic hypochromic anemia on CBC 2. Low serum ferritin (most sensitive for iron deficiency) 3. Low serum iron + elevated TIBC = iron deficiency confirmed 4. Transferrin saturation <16% = iron deficiency (not other causes) ### Iron Study Interpretation in This Case | Parameter | Value in Iron Deficiency | Interpretation | |---|---|---| | Serum ferritin | <12 ng/mL | Depleted iron stores | | Serum iron | ↓ | Low circulating iron | | TIBC | ↑ | Increased iron-binding capacity (compensatory) | | Transferrin saturation | <16% | Confirms iron deficiency | **Clinical Pearl:** Transferrin saturation is calculated as: $Transferrin\ saturation = \frac{Serum\ iron (\mu g/dL)}{TIBC (\mu g/dL)} \times 100$ A saturation <16% is diagnostic of iron deficiency and helps exclude other causes of microcytic anemia (thalassemia, anemia of chronic disease, sideroblastic anemia). **Mnemonic: TIBC-SAT** — TIBC elevated, Saturation <16% = Absolute iron deficiency