The best way to evaluate a low-normal B12 level is to check serum methylmalonic acid and homocysteine levels1 (strength of recommendation [SOR]: B, based on consistent level 2 or 3 studies). Give 1 or 2 mg of oral vitamin B12 a day if levels are marginally low and either methylmalonic acid or both methylmalonic acid and homocysteine are elevated (SOR: A)
