Macro-B(12) masking B(12) deficiency

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.