In ruling out vitamin B12 deficiency, if B12 is low normal or low, check homocysteine and methylmalonic acid levels.  Homocysteine adds a methyl group to create methionine in a reaction catalyzed by vitamin b12 and folate.  As a result, if both homocysteine and MMA are elevated, this strongly suggests vitamin B12 deficiency.  MMA is converted to succinyl-CoA in a reaction catalyzed by vitamin B12 but not folate.   If vitamin B12 deficiency is confirmed, test anti-intrinic factor antibodies to see if the cause is pernicious anemia. If not, keep hunting for the cause.   Keep in mind that oral contraceptives, multiple myeloma, HIV, pregnancy can give spuriously low vitamin B12 levels.  In addition, dilantin can interfere with folate absorption and metabolism, causing an elevated MCV.  If the patient is on dilantin and has an elevated mcv, check a folate level. Once true deficiency is confirmed, the cause must be examined