Vitamin B12 is an essential vitamin in the formation of red blood cells. Vitamin B12 deficiency can cause anemia (fatigue, pale skin, dizziness, etc.) characterized by large red blood cells (macrocytic anemia) or cause nervous system problems (tingling, memory loss, etc.). A vitamin B12 test can be done alone, but it is traditionally combined with a folic acid test.
High levels of vitamin B12 have no particular clinical significance. Low levels of vitamin B12 with the appropriate signs and symptoms confirm anemia (macrocytic or megaloblastic), which is treated with oral supplements or injections of vitamin B12. Normal levels of vitamin B12 generally indicate that a person does not have a vitamin B12 or folic acid deficiency and, therefore, the signs and symptoms observed are due to other causes. Vitamin B12 reserves are important, and normal blood levels may not adequately reflect low reserves. When vitamin B12 deficiency is suspected, but the measured rate is normal, other tests such as a methylmalonic acid test may be used.