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?

Question

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?

Accepted Answer

C. Serum transferrin saturation and TIBC. ## 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: $Transferrin\ saturation = \frac{Serum\ iron (\mu g/dL)}{TIBC (\mu g/dL)} 	imes 100$

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

Answer

A. Hemoglobin electrophoresis

Answer

B. Soluble transferrin receptor (sTfR) level

Answer

D. Peripheral blood smear examination

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

Iron Study Interpretation in This Case

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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