Diagnosis of cobalamin deficiency I: Usefulness of serum methylmalonic acid and total homocysteine concentrations

The serum cobalamin assay is the primary diagnostic test for cobalamin deficiency. It appears to be an excellent screening test since most patients with clinically confirmed cobalamin deficiency have low levels. Recent studies indicate that the clinical picture of cobalamin deficiency is much more diverse than previously believed. It is also apparent that many patients with low serum cobalamin concentrations are not cobalamin deficient. Thus, there is a need for additional diagnostic tests to further distinguish patients with low serum cobalamin levels who are actually cobalamin deficient and will benefit from lifetime treatment from those who are not deficient and will not benefit. Serum levels of methylmalonic acid and total homocysteine have been shown to be markedly elevated in most patients with cobalamin deficiency, and total homocysteine concentrations are markedly elevated in most patients with folate deficiency. The levels of these metabolites fall to normal if these patients are treated with the appropriate vitamin but remain essentially unchanged if the wrong vitamin is administered. These observations demonstrate that serum methylmalonic acid and total homocysteine levels are useful in diagnosing patients with cobalamin and folate deficiency and in distinguishing between these two vitamin deficiencies.