HoloTC in Serum

Low holotranscobalamin reflects reduced cell-available vitamin B12 and can be an early marker of B12 deficiency, often before total serum B12 becomes clearly low.

Common causes include low intake, especially vegan or low-animal-food diets without reliable B12 supplementation, and malabsorption from autoimmune gastritis/pernicious anemia, atrophic gastritis, coeliac disease, bariatric surgery, gastrectomy, or terminal ileal disease/resection. Long-term metformin, PPIs, H2 blockers, and nitrous oxide use can also contribute.

High holotranscobalamin most commonly reflects recent B12 supplementation, including fortified products (like "energy" drinks), and usually does not indicate a problem when this explains the result.

If persistently high without supplementation, possible causes include reduced clearance in kidney impairment, anorexia nervosa with liver involvement, liver disease or alcohol-related liver stress, inflammation, infection, autoimmune disease, or rarely assay interference such as macro-B12/macro-transcobalamin. Hematologic disorders and solid cancers are better established causes of high total B12 than isolated high holoTC, so interpretation should include total B12, MMA, homocysteine, CBC, kidney, liver, and inflammatory markers.