Subtle and atypical cobalamin deficiency states.

Evidence for cobalamin (vitamin B12) deficiency usually involves some combination of low serum cobalamin levels, clinical abnormalities (classically, megaloblastic anemia and neurologic defects), metabolic abnormalities, and response to therapy. However, cobalamin deficiency may often display few of the expected clinical findings. Identification of the underlying cause is also important in the diagnosis of deficiency, and its value may be particularly great when the expression of deficiency is subtle. The cause of cobalamin deficiency is usually malabsorptive, but may sometimes be limited to malabsorption of food cobalamin while free cobalamin is absorbed normally. Nongastroenterologic entities may sometimes also be found. All of these considerations allow the proposal of four patterns of cobalamin deficiency. The first type is classical deficiency; typical megaloblastic anemia with or without neurologic dysfunction occurs because of classical cobalamin malabsorption such as lack of intrinsic factor (pernicious anemia). The second type consists of classical cobalamin malabsorption in which the cobalamin deficiency is expressed subtly rather than in classical fashion. There is no megaloblastic anemia and sometimes the only evidence of deficiency may be metabolic. In the third type, cobalamin deficiency is expressed classically but is attributable to a subtle or atypical cause, such as food-cobalamin malabsorption. In the fourth type, deficiency is both expressed subtly and arises from subtle or atypical causes. Such presentations require further investigation but are a challenging expansion of our understanding and recognition of cobalamin deficiency.