What could cause elevations of methylmalonic acid (MMA) with normal vitamin B12 in a nonvegetarian adult? Should B12 supplements be used?
—Santosh D. Kaushal, MD, Rockville, Md.

An elevated MMA is a very sensitive and specific marker of B12 deficiency. There is evidence that up to 10% of individuals who have normal or low normal B12 values (between 150 and 400 pg/mL) may develop neuropsychiatric sequelae of B12 deficiency without any evidence of megaloblastic anemia. The MMA test can identify this population of functionally B12-deficient individuals. In these patients, oral cyanocobalamin 1-2 mg/day will normalize the MMA and hopefully improve their symptoms. MMA levels can be falsely elevated in renal insufficiency and hypovolemia, in which case levels do not correct with vitamin B12 repletion.
—Susan Kashaf, MD, MPH