Methylmalonic Acid, Serum or Plasma is a specialized test that measures methylmalonic acid levels to evaluate vitamin B12 deficiency and certain inherited metabolic disorders. MMA is a metabolic byproduct that accumulates when vitamin B12 is deficient, as B12 is required for its conversion to succinate. This test is particularly valuable for detecting early vitamin B12 deficiency, even when serum B12 levels appear normal, and for monitoring response to B12 supplementation. Elevated MMA levels can indicate vitamin B12 deficiency, which can cause neurological symptoms, anemia, and cognitive impairment. This test is essential for evaluating patients with suspected B12 deficiency, unexplained neurological symptoms, or certain inherited metabolic disorders.