Letters To The Editor

In reply: Vitamin B12 deficiency

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In Reply: We thank Dr. Phillips for his inquiry.

In general, serum vitamin B 12 concentrations vary greatly, and we acknowledge that serum vitamin B 12 may be normal in up to 5% of patients with documented B 12 deficiency. 1 In a prospective study of 1,599 patients, Matchar et al 2 demonstrated that a single vitamin B 12 level less than 200 pg/mL had a specificity greater than 95% at predicting vitamin B 12 deficiency. 2 We acknowledge that additional metabolite testing is necessary in equivocal cases in which the vitamin B 12 level is between 200 and 300 pg/mL, which is often considered to be the normal range, but the patient has symptoms of vitamin B 12 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 B 12 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 B 12 deficiency.

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