Clinical recommendation	Evidence rating	References
Obtain a vitamin B12 level for every patient with an elevated mean corpuscular volume.	C	1, 3–5, 19
Evaluate peripheral smear for megaloblastosis and perform a reticulocyte count in patients with suspected macrocytosis.	C	1, 3–5, 19
Order methylmalonic acid and homocysteine levels if vitamin B12 level is borderline low (i.e., 100 to 400 pg per mL [74 to 295 pmol per L]).	C	8
Oral vitamin B12 may be as effective as intramuscular therapy for vitamin B12 deficiency.	B	21
Obtain red blood cell folate level if other etiologies are not found (serum folate levels may be misleading).	C	25, 26