Until recently, it has been difficult to test for vitamin B12 deficiency. An inability to properly absorb vitamin B-12 causes pernicious anemia and leads to fatigue and neurological problems. Physicians lacked a safe and simple way to test their patients for poor vitamin B12 absorption. In humans, only two enzymatic reactions are dependent on vitamin B12. In the first reaction, methylmalonic acid is converted to succinyl-CoA by using vitamin B12 as a cofactor. Vitamin B12 deficiency can lead to increased levels of serum methylmalonic acid. Homocysteine is converted to methionine by using vitamin B12 and folic acid as cofactors. A deficiency of vitamin B12 or folic acid may lead to increased homocysteine levels. An understanding of the vitamin B12 absorption cycle helps clarify the potential causes of deficiency. The acidic environment of the stomach helps to breakdown the vitamin B12 that is bound to food. The intrinsic factor that is released by parietal cells in the stomach binds to vitamin B12 in the duodenum.
Extensive research has resulted in a more accurate and safer test for vitamin B12 deficiency.The new test involves drinking a small amount of vitamin B12 labeled with radioactive carbon 14, and then extracting a drop of blood. The amount of carbon 14 labeled vitamin B12 in the blood sample is measured with an accelerator mass spectrometer that can count single atoms of carbon 14. The radiation dose involved is equivalent to that received on a cross-country flight.