Unexplained Hypercobalaminemia: A Rare Diagnostic Pitfall.

Persistently elevated serum vitamin B12 may suggest underlying pathology once analytical interferences are excluded. Here, we report a case of 56-year-old female with high vitamin B12 levels incidentally noted by primary care provider. Repeat testing with different assay method over time demonstrated consistently high values, prompting further evaluation. Hematology-oncology provider initiates malignancy workup due to well-documented association between neoplastic process and elevated vitamin B12 levels. Concurrently, clinical laboratory was contacted to rule out potential assay interference related to macro-vitamin B12 complexes. A polyethylene glycol precipitation study demonstrated markedly low vitamin B12 recovery, supporting the presence of a macro-vitamin B12 complex as the cause of spurious elevation. This case report highlights the importance of excluding macro-vitamin B12 interference prior to initiating extensive malignancy workup and the lack of evidence-based guidelines for the laboratory assessment of macro-complexes.