To the Editor.
—The recent article by Reynolds et al1 on potential association of vitamin B12 "deficiency" and multiple sclerosis (MS) is of considerable interest to patients with MS and physicians involved in their care. The significance of this observation, however, remains unclear because convincing evidence of a vitamin B12deficiency in patients with MS was not documented by the authors. Although the serum vitamin B12 assay is an extremely sensitive test for a vitamin B12 deficiency, it lacks specificity. It has been established that many patients with low serum B12 levels do not have a documentable cobalamin deficiency state. The currently available confirmatory tests for biologically significant vitamin B12 deficiency include determination of serum methylmalonate and homocysteine levels, which have been shown to be elevated in the vast majority of vitamin B12—deficient patients with objective hematologic or neuropsychiatric changes attributable