Vitamin B deficiency from the perspective of a practicing hematologist.

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