A 48-year-old woman on long-term omeprazole (40 mg daily for 5 years) for GERD presents with fatigue, paresthesias, and macrocytic anemia (Hb 8.2 g/dL, MCV 108 fL). Serum B12 is 180 pg/mL (normal 200–900). Which investigation is most appropriate to confirm the etiology of her B12 deficiency?

## Diagnosis: PPI-Induced Vitamin B12 Deficiency **Key Point:** Long-term PPI use (>1 year) impairs gastric acid secretion, reducing the release of B12 from food protein and impairing intrinsic factor production. This leads to food-bound B12 malabsorption (not pernicious anemia). The distinction is critical: PPI-induced B12 deficiency is due to achlorhydria affecting food-bound B12 absorption, NOT autoimmune destruction of parietal cells. **High-Yield:** **Serum methylmalonic acid (MMA) and homocysteine** are the most appropriate investigations to confirm functional B12 deficiency and assess its metabolic impact. These are sensitive markers of B12 status and help differentiate true B12 deficiency from other causes of macrocytic anemia. ### Why MMA and Homocysteine? 1. **Metabolic Markers:** Both MMA and homocysteine are elevated in true B12 deficiency because: - B12 is a cofactor for methylmalonyl-CoA mutase (converts MMA → succinyl-CoA) - B12 is a cofactor for methionine synthase (converts homocysteine → methionine) 2. **Sensitivity:** More sensitive than serum B12 alone, especially in early or marginal deficiency. 3. **Specificity:** Confirms that the low B12 level is functionally significant and causing metabolic derangement. 4. **Clinical Pearl:** Normal MMA and homocysteine with low B12 suggests false deficiency (e.g., lab error, transcobalamin deficiency). ### Diagnostic Approach Table | Investigation | PPI-Induced B12 Deficiency | Pernicious Anemia (Autoimmune) | |---|---|---| | **Serum B12** | Low | Low | | **Intrinsic Factor Antibody** | Negative | Positive (60–70%) | | **Parietal Cell Antibody** | Negative | Positive (90%) | | **MMA & Homocysteine** | Elevated | Elevated | | **Gastric pH** | High (>6) due to achlorhydria | Normal or low | | **Schilling Test (if done)** | Abnormal without IF, corrects with IF | Abnormal without IF, does NOT correct with IF | | **Gastric Biopsy** | Atrophic gastritis, no autoimmune inflammation | Autoimmune gastritis with plasma cell infiltrate | **Mnemonic:** **MAH** = **M**ethylmalonic acid, **A**nd **H**omocysteine — the functional markers of B12 deficiency. ### Why NOT the Other Options? **Intrinsic Factor & Parietal Cell Antibodies:** - These are negative in PPI-induced deficiency because the problem is achlorhydria (reduced acid production), not autoimmune destruction of parietal cells. - They are positive in pernicious anemia (autoimmune gastritis), which this patient does not have. - These tests confirm the mechanism of deficiency, not the presence of deficiency. **Schilling Test:** - Largely obsolete in modern practice (rarely available). - While it can differentiate malabsorption from intrinsic factor deficiency, it does not confirm B12 deficiency itself—serum B12 and MMA/homocysteine are more practical. - Requires radioactive cobalt-57, which is not routinely available. **Gastric Biopsy:** - Invasive and not the investigation of choice for confirming B12 deficiency. - Useful for assessing the degree of atrophic gastritis but does not confirm functional B12 deficiency. - Reserved for specific indications (e.g., suspected gastric malignancy, refractory anemia). **Clinical Pearl:** In a patient on long-term PPIs with macrocytic anemia and low B12, the combination of **elevated MMA and homocysteine** confirms that the B12 deficiency is metabolically significant and requires treatment, regardless of the underlying mechanism (PPI-induced vs. autoimmune). [cite:Harrison 21e Ch 102]