Macro-Vitamin B12 as Cause of Falsely Elevated Cobalamin Levels.

Introduction: High blood concentrations of vitamin B12 are often caused by over-supplementation. However, there are instances in which augmented vitamin B12 levels are seen in the absence of supplements. Macro-vitamin B12 is an underrated cause of supra-physiological cobalamin plasma levels.

Case Description: A 70-year-old man was referred to an ambulatory internal medicine centre because of high vitamin B12 levels yet he denied taking supplements. An X-ray showed a tumour in the right upper lobe of the lung, which triggered further examinations. An MRI scan of the brain came back normal as well as a CT scan of the abdomen, and colonoscopy. The pulmonologist requested a PET-CT scan, which showed an isolated 18-FDG uptake in the area of the lung mass that was detected earlier. The patient underwent surgery with adjuvant cis-platinum and gemcitabine and is still making good progress. The vitamin B12 levels persisted after successful treatment of lung adenocarcinoma; determination of vitamin B12 after PEG (polyethylene glycol) precipitation showed normal concentrations.

Discussion: A high vitamin B12 plasma concentration in the absence of vitamin supplementation can be a daunting diagnostic problem for the internist, as there are several possible underlying causes. In this case the diagnosis of lung carcinoma was made, the patient was treated appropriately, yet this pathology had no correlation with the cobalamin levels.

Conclusion: A high vitamin B12 concentration can be the impetus of thorough medical inquiries. Internists should be careful not to forget macro-vitamin B12 as a possible source of falsely elevated vitamin B12 values.

Learning Points: When encountering an otherwise unexplainable B12 hypervitaminosis, the diagnosis of macro-vitamin B12 should be taken into account to avoid unnecessary extensive medical examinations.PEG precipitation can distinguish between a 'real' high vitamin B12 and macro-vitamin B12.A high vitamin B12 concentration is no guarantee for adequate cobalamin storage. In case of suspicion, a vitamin B12 measurement after PEG precipitation should be considered.