Vitamin B12 deficiency from the perspective of a practicing haematologist.

Ralph Green
Department of Pathology and Laboratory Medicine, University of California, Davis, Sacramento, CA

B12 deficiency is the leading cause of megaloblastic anemia, and although more common in the elderly, can occur at any age. Clinical disease caused by B12 de- ficiency usually connotes severe defi- ciency, resulting from a failure of the gastric or ileal phase of physiological B12 absorption, best exemplified by the autoimmune disease pernicious anaemia.There are many other causes of B12deficiency, which range from severe to mild. Mild deficiency usually results from failure to render food B12 bioavailable or from dietary inadequacy. Although rarely resulting in megaloblastic anemia, mild deficiency may be associated with neu- rocognitive and other consequences. B12 deficiency is best diagnosed using a combination of tests because none alone is completely reliable. The features of B12deficiency are variable and may be atypical. Timely diagnosis is important, and treatment is gratifying. Failure to diagnose B12 defi- ciency can have dire consequences, usually neurological. This review is written from the perspective of a practicing hematologist. (Blood. 2017;129(19):2603-2611)