A 52-year-old woman with pernicious anemia presents with macrocytic anemia (Hb 7.2 g/dL, MCV 108 fL) and neurological symptoms. Which investigation is most specific for confirming vitamin B12 deficiency as the cause of megaloblastosis?

Explanation

## Investigation of Choice in B12 Deficiency **Key Point:** Methylmalonic acid (MMA) and homocysteine are the most specific functional markers of B12 deficiency because they directly reflect B12-dependent enzymatic activity. ### Why MMA and Homocysteine? Vitamin B12 is a cofactor for two critical enzymes: 1. **Methylmalonyl-CoA mutase** — converts methylmalonyl-CoA to succinyl-CoA (TCA cycle) 2. **Methionine synthase** — remethylates homocysteine to methionine When B12 is deficient, both pathways are blocked, causing: - **↑ Methylmalonic acid** (most specific for B12 deficiency) - **↑ Homocysteine** (elevated in both B12 and folate deficiency) ### Diagnostic Hierarchy | Investigation | Specificity | Sensitivity | Use Case | |---|---|---|---| | **Serum B12 level** | Moderate | Moderate | Screening; affected by food-bound B12 | | **MMA + Homocysteine** | **Very high** | **Very high** | **Gold standard for functional B12 status** | | **Schilling test** | High | High | Differentiates pernicious anemia from malabsorption (rarely done now) | | **Bone marrow biopsy** | High | High | Confirms megaloblastosis; does not identify cause | **High-Yield:** MMA is **elevated ONLY in B12 deficiency**, whereas homocysteine is elevated in both B12 and folate deficiency. This makes MMA the most specific single marker. ### Clinical Pearl In a patient with neurological symptoms (subacute combined degeneration risk), MMA + homocysteine should be checked even if serum B12 is borderline-low (200–300 pg/mL), because functional deficiency may exist despite "normal" serum levels. [cite:Harrison 21e Ch 100]