115 Erroneously normal active B12 level in a case of subacute combined degeneration of the cord

Objectives Vitamin B12 is crucial for neurologic function, red blood cell production, and DNA synthesis. Deficiency can lead to a wide spectrum of haematologic and neuropsychiatric disorders including subacute combined degeneration of the cord. This report presents a case of a 47 y.o female who presented with subacute combined degeneration of the spinal cord with a normal active B12 level. Results MRI demonstrated high T2 signal throughout the dorsal columns of the cervical and upper thoracic cord without enhancement consistent with subacute combined degeneration of the cord. Her blood count revealed a mild macrocytic anaemia. Total vitamin B12 was <80 pmol/L and active B12 was >128 pmol/L, confirmed on repeat testing. Functional vitamin B12 deficiency was confirmed by an elevated homocysteine level of 38.6 umol/L (reference range 4.4 to 13.6 umol/L) and elevated serum methylmalonic acid of 20.75 umol/L (reference range <0.32 umol/L). Intrinsic factor and gastric parietal cell antibodies were detected. Conclusion Our patient had a clinical presentation consistent with B12 deficiency with an erroneously high active B12 level. Functional assays confirmed B12 deficiency, and a serological diagnosis of pernicious anaemia was made. This case illustrates the importance of not relying on any single test to exclude B12 deficiency.