Vitamin B12 — Normal Range & Interpretation

Clinical Context

Vitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, red blood cell maturation, and myelin formation in peripheral and central nerves. Serum B12 screens for deficiency states that produce macrocytic anemia and neurologic symptoms, and it guides supplementation decisions in at-risk populations including older adults, vegans, and patients on long-term metformin or proton pump inhibitors.

Depressed B12 results from pernicious anemia (autoimmune loss of intrinsic factor), atrophic gastritis, ileal disease or resection, chronic PPI or H2 blocker use, metformin therapy, and strict vegan diets. Classic findings include macrocytic anemia with hypersegmented neutrophils, glossitis, paresthesias, ataxia, and subacute combined degeneration of the spinal cord. Elevated B12 points to excessive supplementation, liver disease, myeloproliferative disorders, or certain malignancies.

Classic AANP vignette: macrocytic anemia on the CBC, with the distinction between B12 and folate deficiency (folate corrects anemia but not neuropathy), and identification of high-risk patients such as long-term metformin users and post-gastric-bypass patients. Expect questions linking dorsal column signs, decreased vibratory sense, and positive Romberg to B12 deficiency, and questions on checking methylmalonic acid when serum B12 falls in the borderline range.

Related Labs

• HbA1c — %
• ESR — mm/hr
• CRP — mg/dL
• hs-CRP — mg/L
• Uric Acid — mg/dL

Sources

• MedlinePlus