—We thank Goodkin and colleagues for their interest in our article1 on 10 patients with multiple sclerosis (MS) associated with vitamin B12 deficiency, and we agree with their view that the serum vitamin B12 assay may be an unreliable guide to a vitamin B12 deficiency state. Indeed, not only may some patients with a low serum vitamin B12 level not have a vitamin B12 deficiency but others with vitamin B12 levels above 200 pg/mL may be severely deficient.2 For example, we have studied one patient with a high serum vitamin B12 level (greater than 2000 pg/mL) who had subacute combined degeneration related to a severe vitamin B12 deficiency state, as reflected in a megaloblastic anemia, low red blood cell vitamin B12 level, high plasma homocysteine level, and hematologic and neurologic response to vitamin B12 treatment.3
Reynolds EH, Linnell JC, Faludy JE, Bottiglieri T. Biologically Significant Serum Vitamin B12 Deficiency in Multiple Sclerosis Inadequately Documented-Reply. Arch Neurol. 1992;49(7):683–684. doi:10.1001/archneur.1992.00530310021006
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