What Is Vitamin B12 in a Blood Test?

Vitamin B12 (cobalamin) is essential for DNA synthesis, red blood cell formation, neurological function, and homocysteine metabolism.

Normal Range

• 200–900 pg/mL
• Below 200: deficient
• 200–400: borderline — consider MMA and homocysteine testing
• Above 400: generally adequate

Deficiency Symptoms

• Hematologic: macrocytic anemia, fatigue, pallor
• Neurologic: peripheral neuropathy, numbness/tingling, balance problems
• Psychiatric: depression, memory impairment
• GI: glossitis (sore tongue)

Deficiency Causes

• Pernicious anemia — autoimmune destruction of intrinsic factor
• Vegan/vegetarian diet
• Malabsorption — celiac, Crohn’s, gastric bypass
• Medications — metformin, PPIs (omeprazole)
• Age — reduced stomach acid
• Nitrous oxide use

Beyond Serum B12

• If borderline, check methylmalonic acid (MMA) and homocysteine
• Anti-intrinsic factor antibodies help diagnose pernicious anemia