A serum vitamin B-12 level less than 150 nanograms per liter is evidence of vitamin B-12 deficiency, states Mayo Clinic. Patients with vitamin B-12 levels between 150 and 400 nanograms per liter should be evaluated by further tests.
Further tests include checking homocysteine levels and methylmalonic acid levels. In vitamin B-12 deficiency, homocysteine and methylmalonic acid levels are raised, according to Mayo Clinic.
Vitamin B-12 is necessary for blood cell production and neuron function, states Mayo Clinic. A serum vitamin B-12 level that is less than 180 nanograms per liter may cause anemia and neuropathies.
Vitamin B-12 deficiency may be due to a lack of intrinsic factor secretion by the gastric mucosa or due to malabsorption by the intestines, reports Mayo Clinic. For people with vitamin B-12 levels less than 150 nanograms per liter, tests for antibodies to intrinsic factor are recommended. If the vitamin B-12 level is between 150 and 400 nanograms per liter, and the methylmalonic acid level is greater than 0.04 units per liter, tests for antibodies to intrinsic factor are also recommended. If the vitamin B-12 level is less than 150 nanograms per liter, and the intrinsic factor antibody test is negative or indeterminate, testing gastrin levels is recommended.
People who have taken vitamin B-12 supplements may have misleading results, states Mayo Clinic. Conditions such as pregnancy, smoking, drinking alcohol, and taking aspirin or oral contraceptives decrease vitamin B-12 levels.