Combined indicator of B12 status (cB12 status)

Low B12 most often reflects inadequate intake (vegan or vegetarian diets, chronic alcohol use) or malabsorption.

Malabsorption causes include pernicious anemia (autoimmune loss of intrinsic factor), celiac disease, Crohn’s disease, atrophic gastritis, low stomach acid, prior gastric or ileal surgery, inflammatory bowel disease, and H. pylori or small intestinal bacterial overgrowth.

Common drug causes: long-term metformin, proton pump inhibitors, and H2 blockers.

High B12 is usually caused by B12 supplements or injections, which is generally not a concern since B12 is not considered toxic.

When not intake-related, elevated B12 can accompany liver disease (cirrhosis, hepatitis, hepatocellular carcinoma, or liver metastases), kidney failure, diabetes, myeloproliferative and other hematological disorders (leukemia, polycythemia vera, hypereosinophilic syndrome), and other cancers (colon, pancreas, breast, stomach).

Elevated serum B12 can also coexist with functional B12 deficiency, since the test also measures B12 bound to transport proteins that is not taken up by cells.

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.

Low MMA is generally not a concern — lower values indicate good functional B12 status with no established lower limit of harm.

High MMA most often reflects functional vitamin B12 deficiency — the main and usually correctable cause, as MMA accumulates when B12-dependent metabolism of methylmalonyl-CoA stalls. It is more specific to tissue B12 status than serum B12.

Non-B12 causes: reduced kidney function (the most important confounder, from decreased clearance), dehydration, hypothyroidism, pregnancy, and gut bacterial overgrowth (SIBO). Markedly high levels in infants or children suggest inherited methylmalonic acidemia.

homocysteine is a substrate for glutathione and cysteine synthesis, very low levels may limit these antioxidant and detoxification pathways. Low levels have also been associated with idiopathic peripheral neuropathy.

High homocysteine most often reflects insufficient folate, vitamin B12, or vitamin B6 — the main and usually correctable cause, including with vegetarian or vegan diets (lower B12).

Other contributors: impaired kidney function (reduced clearance), hypothyroidism, MTHFR and related genetic variants (MTR, CBS), advancing age, smoking, alcohol, chronic stress, and certain medications (methotrexate, metformin, cholestyramine, antiepileptics). Markedly high levels suggest inherited homocystinuria.

Low folate usually reflects inadequate intake — diets low in leafy greens and legumes, or losses from prolonged cooking (folate is heat-labile).

Other causes: malabsorption (celiac disease, IBD, bariatric surgery), chronic alcohol use, increased demand (pregnancy, lactation, chronic hemolysis, exfoliative skin disorders), and folate-depleting medications (methotrexate, sulfasalazine, trimethoprim, phenytoin and other anticonvulsants, metformin).

High folate is most often intake-driven — recent folic acid or B-complex supplementation, fortified foods, or a high-folate meal, since serum folate reflects intake over the preceding days.

It can also accompany vitamin B12 deficiency (the "methyl-folate trap"), where folate accumulates as methyl-THF and can mask the underlying B12 deficiency. Reduced kidney clearance can also raise levels.