Guidelines suggest serum B12 measurement in patients with suspected dementia in order to screen for severe B12 deficiency as a potentially treatable cause of cognitive decline. There remains a concern that some individuals with more modestly low levels of B12 may also experience cognitive decline that may be made clinically worse by high levels of serum folate, a more common occurrence in the United States since the introduction of mandatory folic acid grain supplementation.
A recent study aimed to examine the relationship between cognitive decline, MRI changes, and levels of B12 and other biochemical markers of B12 deficiency, namely serum homocysteine and methylmalonic acid (MMA). In a study of 6158 persons, both homocysteine and MMA were significantly associated with poorer cognitive testing across multiple domains; B12 levels demonstrated no such association. Access Medicine, October 2011