Vitamin B12

The Vitamin B12 test (serum cobalamin, cyanocobalamin) is used to assess macrocytic or megaloblastic anemia and to investigate neurologic or neuropsychiatric features attributable to cobalamin deficiency. It supports evaluation of malnutrition and malabsorption states such as celiac disease and Crohn disease, assessment in alcoholism, review of prenatal care/nutritional status, and monitoring of treatment for confirmed vitamin B12 deficiency. Markedly increased serum B12 may accompany liver disease or myeloid leukemia. Interpretation should be integrated with clinical findings and related studies, including folate, homocysteine, and complete blood count indices.

Serum measurement of vitamin B12 is a standard approach to evaluate cobalamin status. Dietary sources are predominantly animal-derived foods—meat (notably liver and kidney), fish, eggs, and dairy—with additional intake from certain fortified grain products. Cobalamin is required for nucleic acid synthesis, erythropoiesis, and maintenance of neurologic integrity; deficiency produces megaloblastic anemia and may involve the posterior columns and corticospinal tracts, presenting with distal paresthesias and sensory loss. Causes of deficiency include inadequate intake (eg, prolonged strict vegetarian patterns or starvation), impaired absorption (eg, pernicious anemia from intrinsic factor deficiency, celiac disease, Crohn disease, achlorhydria, or postsurgical reduction of absorptive surface), decreased utilization in liver or kidney disease and with alcohol misuse, and increased requirements (eg, hemolytic anemia, hyperthyroidism, alpha-thalassemia).