A 48-year-old Indian woman with a 10-year history of strict vegetarian diet presents with megaloblastic anemia. A 55-year-old man with chronic atrophic gastritis and intrinsic factor antibodies also has megaloblastic anemia. Which laboratory finding best distinguishes dietary B12 deficiency from pernicious anemia?

Question

Accepted Answer

B. Schilling test showing impaired absorption corrected by intrinsic factor. ## Distinguishing Dietary B12 Deficiency from Pernicious Anemia

**Key Point:** Both dietary B12 deficiency and pernicious anemia (autoimmune B12 deficiency) present with megaloblastic anemia and low B12 levels, but the **Schilling test** definitively differentiates them by identifying the mechanism of deficiency: inadequate intake vs. impaired absorption.

### Comparison Table

| Feature | Dietary B12 Deficiency | Pernicious Anemia |
| --- | --- | --- |
| **Cause** | Inadequate dietary intake | Autoimmune destruction of parietal cells |
| **Intrinsic factor antibodies** | Absent | Present |
| **Parietal cell antibodies** | Absent | Present (60%) |
| **Serum B12** | Low | Low |
| **Methylmalonic acid** | Elevated | Elevated |
| **Homocysteine** | Elevated | Elevated |
| **Schilling test (stage 1)** | Normal absorption (corrected by IF) | Abnormal absorption |
| **Schilling test (stage 2)** | Normal | Corrected by intrinsic factor |
| **Gastric parietal cell atrophy** | Absent | Present |
| **Achlorhydria** | Absent | Present |

### Why Schilling Test Is the Gold Standard?

1. **Mechanism identification:** Stage 1 (B12 alone) shows whether absorption is impaired. Stage 2 (B12 + intrinsic factor) determines if the defect is IF-related.
2. **In dietary deficiency:** Schilling stage 1 is **normal** (absorption is intact); stage 2 is also normal. The problem is intake, not absorption.
3. **In pernicious anemia:** Schilling stage 1 is **abnormal** (impaired absorption); stage 2 is **corrected by IF**, proving the defect is lack of intrinsic factor.
4. **Clinical Pearl:** The Schilling test is the only investigation that directly assesses the mechanism of B12 malabsorption and is diagnostic for pernicious anemia.

**High-Yield:** Schilling test stage 2 correction = pernicious anemia (IF deficiency). No correction = other causes of malabsorption (ileal disease, pancreatic insufficiency).

**Mnemonic:** **SCHILLING-IF** — Schilling test with intrinsic factor correction confirms pernicious anemia.

Answer

A. Serum B12 level < 200 pg/mL

Answer

C. Intrinsic factor antibodies and parietal cell antibodies

Answer

D. Elevated serum methylmalonic acid and homocysteine

Explanation

Distinguishing Dietary B12 Deficiency from Pernicious Anemia

Comparison Table

Why Schilling Test Is the Gold Standard?

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Feature	Dietary B12 Deficiency	Pernicious Anemia
Cause	Inadequate dietary intake	Autoimmune destruction of parietal cells
Intrinsic factor antibodies	Absent	Present
Parietal cell antibodies	Absent	Present (60%)
Serum B12	Low	Low
Methylmalonic acid	Elevated	Elevated
Homocysteine	Elevated	Elevated
Schilling test (stage 1)	Normal absorption (corrected by IF)	Abnormal absorption
Schilling test (stage 2)	Normal	Corrected by intrinsic factor
Gastric parietal cell atrophy	Absent	Present
Achlorhydria	Absent	Present