R-binder deficiency. A clinically benign cause of cobalamin pseudodeficiency.

A patient with R-binder deficiency lacked this cobalamin-binding protein in serum, saliva, gastric juice, and leukocytes, as demonstrated by cobalamin-binding ability and radioimmunoassay. The patient's condition was first detected because of a low serum cobalamin (vitamin B12) level. His level of serum cobalamin, however, which was carried largely by transcobalamin II, was not as decreased as in the initial cases reported and was higher by some assays than by others. The demonstration of normal bone marrow morphological appearance and deoxyuridine suppression test findings, lack of attributable neurological dysfunction, and absence of methylmalonic aciduria make clear that deficiency of R binder, whose function is unknown, did not produce notable disturbance of cobalamin metabolism. This entity is probably not as rare as originally thought. It must be considered in the differential diagnosis of low serum cobalamin levels, even among elderly patients, whose conditions often tend to be automatically diagnosed as pernicious anemia.