Unexplained Elevated Vitamin B12: Consider Macro-B12

Elevated vitamin B12 concentration can be caused by supplementation, liver disease, kidney disease, or myeloid malignancies. Persistent, unexplained elevations of vitamin B12 can raise concern among patients and may lead to invasive diagnostic procedures, including bone marrow biopsy. A potential benign cause of this elevation is macro-B12, a complex of vitamin B12, transcobalamin, and immunoglobulins. Although not bioactive, this complex can cause elevated plasma vitamin B12 concentrations due to reduced clearance.

Polyethylene glycol (PEG) precipitation is a laboratory technique that can be used to support the suspicion of macro-B12. Here, a case is described in which a PEG precipitation could potentially have prevented an unnecessary bone marrow biopsy. In addition, the presence of macro-B12 was studied in a group of patients with and without myeloid malignancies.

Macro-B12 was identified in 24% of 72 individuals with vitamin B12 > 1476 pmol/L. In one of these patients, a functional vitamin B12 deficiency was confirmed by an elevated methylmalonic acid (MMA) concentration. Macro-B12 was not detected in 8 patients with a myeloid malignancy.

These findings suggest that, in patients with persistently elevated vitamin B12 concentrations and a low suspicion of a myeloid malignancy, PEG precipitation may help to explain the elevated vitamin B12 and prevent unnecessary diagnostic procedures including bone marrow punctures.