In reply: Vitamin B₁₂ deficiency (July 2015)

We thank Dr. Phillips for his inquiry. In general, serum vitamin B₁₂ concentrations vary greatly, and we acknowledge that serum vitamin B₁₂ may be normal in up to 5% of patients with documented B₁₂ deficiency.¹ In a prospective study of 1,599 patients, Matchar et al² demonstrated that a single vitamin B₁₂ level less than 200 pg/mL had a specificity greater than 95% at predicting vitamin B₁₂ deficiency.² We acknowledge that additional metabolite testing is necessary in equivocal cases in which the vitamin B₁₂ level is between 200 and 300 pg/mL, which is often considered to be the normal range, but the patient has symptoms of vitamin B₁₂ deficiency such as dementia and unexplained macrocytosis, and neurologic symptoms.³

Based on the patient’s symptoms of neuropathy and fatigue in conjunction with a vitamin B₁₂ level well below 200 pg/mL, we believe that the diagnosis can be made.^2,3 Nonetheless, although we did not mention it in our article, we did indeed send for a methylmalonic acid measurement at the time of the initial evaluation, and the level was elevated at 396 nmol/L (normal 87–318 nmol/L), further confirming her vitamin B₁₂ deficiency.

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