During the workup of a 52-year-old man with macrocytic anemia (MCV 108 fL, Hb 7.2 g/dL), the following investigations are performed: serum B12 level 180 pg/mL (low-normal), serum folate 2.1 ng/mL (low), methylmalonic acid (MMA) elevated, homocysteine elevated, and bone marrow shows hypersegmented neutrophils. All of the following support a diagnosis of vitamin B12 deficiency EXCEPT:

## Vitamin B12 Deficiency — Diagnostic Workup ### Biochemical Markers of B12 Deficiency **Key Point:** Methylmalonic acid (MMA) and homocysteine are the two most specific functional markers of B12 deficiency. They differentiate B12 deficiency from folate deficiency, which affects only homocysteine metabolism. | Marker | B12 Deficiency | Folate Deficiency | Normal Range | |--------|----------------|-------------------|---------------| | Serum B12 | ↓ | Normal | 200–900 pg/mL | | Methylmalonic Acid | ↑ | Normal | < 0.4 µmol/L | | Homocysteine | ↑ | ↑ | < 15 µmol/L | | Serum Folate | Normal or ↓ | ↓ | > 5.4 ng/mL | ### Why Low Serum Folate Does NOT Support B12 Deficiency **Warning:** This is a critical distinction. Low serum folate is **not** specific to B12 deficiency — it is the hallmark of **folate deficiency**. The two conditions can coexist, but low folate alone does not support B12 deficiency. **Mnemonic: MMA-B12 (MMA is specific to B12)** — Elevated methylmalonic acid is pathognomonic for B12 deficiency because: - B12 is a cofactor for **methylmalonyl-CoA mutase** - Without B12, methylmalonic acid accumulates and is excreted in urine - Folate deficiency does **NOT** cause elevated MMA **High-Yield:** The three findings that **specifically support B12 deficiency** are: 1. **Elevated MMA** ✓ (B12-specific metabolic marker) 2. **Elevated homocysteine** ✓ (B12 required for remethylation of homocysteine) 3. **Hypersegmented neutrophils** ✓ (megaloblastic change from impaired DNA synthesis) ### Clinical Pearl In a patient with macrocytic anemia and low-normal B12, the presence of elevated MMA and homocysteine confirms **functional B12 deficiency** (also called "early" or "subclinical" B12 deficiency). Low serum folate in this context suggests **concurrent folate deficiency** (common in malnutrition, alcoholism, or malabsorption), not B12 deficiency. The diagnosis rests on the metabolic markers (MMA, homocysteine) and bone marrow morphology, not folate levels. [cite:Harrison 21e Ch 406]