Macro-B 12 masking B 12 deficiency

Abstract

In clinical practice, the finding of an elevated serum B 12 concentration is often the consequence of supplementation with B 12 in either oral form or injections. Also, elevated serum B 12 may be associated with underlying disorders, like liver diseases or a (haematologic) malignancy. Only a few studies have shown that it may also be the consequence of complex formation of B 12 -vitamin binding proteins with immunoglobulins, the so-called macro-B 12. We describe a young woman who previously was diagnosed with B 12 deficiency, and in whom, after cessation of B 12 injection treatment, neurologic symptoms re-appeared, and despite this, repeatedly elevated serum B 12 concentrations above the upper limit of the assay were found. We demonstrated that this was caused by the presence of macro-B 12, which not only resulted in erroneous and longstanding elevated serum B 12, but also masked her underlying B 12 deficiency.