A 38-year-old woman from rural Maharashtra presents with progressive paresthesias in both feet, difficulty walking, and loss of vibration sense in the lower limbs. She reports a strict vegetarian diet with no dairy or egg intake for the past 8 years. Serum cobalamin level is 180 pg/mL (normal >200). Which investigation would be most specific for confirming vitamin B12 deficiency and differentiating dietary from malabsorption causes?

Explanation

## Investigation of Choice for B12 Deficiency Etiology **Key Point:** The Schilling test is the gold standard for distinguishing dietary B12 deficiency from pernicious anemia (malabsorption). It works by measuring urinary excretion of radiolabeled cyanocobalamin after oral and parenteral doses. ### How the Schilling Test Works 1. **Stage 1 (without intrinsic factor):** Oral ⁵⁷Co-labeled B12 is given; if urinary excretion is low (<7% of dose), malabsorption is present. 2. **Stage 2 (with intrinsic factor):** If Stage 1 is abnormal, repeat with intrinsic factor. Correction indicates pernicious anemia; persistent low excretion indicates pancreatic insufficiency or terminal ileal disease. ### Why This Patient Needs the Schilling Test **Clinical Pearl:** This patient has borderline serum B12 (180 pg/mL) with neurological signs (subacute combined degeneration). The 8-year strict vegan diet raises suspicion for dietary deficiency, but the neurological manifestations warrant confirmation of the mechanism. - **Dietary B12 deficiency:** Schilling Stage 1 normal → Stage 2 normal (absorption intact). - **Pernicious anemia:** Schilling Stage 1 abnormal → Stage 2 corrects with intrinsic factor. ### Comparison of B12 Diagnostic Tests | Investigation | Specificity | When to Use | Limitation | |---|---|---|---| | **Serum B12** | Low | Screening only | Borderline results (150–300) are inconclusive | | **Methylmalonic acid (MMA) & homocysteine** | High | Functional B12 status | Elevated in both deficiency and renal impairment; not etiological | | **Intrinsic factor / parietal cell antibodies** | Very high for pernicious anemia | Confirms autoimmune cause | Negative in 50% of pernicious anemia; doesn't help dietary cases | | **Schilling test** | Very high | Distinguishes dietary from malabsorption | Requires nuclear medicine; rarely available now; gold standard | **High-Yield:** In modern practice, if serum B12 is borderline and MMA/homocysteine are elevated, proceed directly to antibody testing (IF, parietal cell) or empirical B12 supplementation. However, for NEET PG exams, the Schilling test remains the textbook answer for etiology determination. **Warning:** Do not confuse the Schilling test with serum B12 or folate levels — those are screening tests, not etiological tests.