A vitamin B12 level test measures the amount of B12 in your blood. Vitamin B12 is a B complex vitamin that is necessary for normal blood cell formation, tissue and cellular repair and DNA synthesis. Source-Vitamin B12 is found in animal products such as meat, shellfish, milk, cheese, and eggs. Why it is done- It helps to diagnose the cause of certain type of anemia such as megaloblastic anemia.It also helps to find the cause of dementia or other nervous system symptoms, such as tingling or numbness of the arms or legs (peripheral neuropathy). To See if vitamin B12 deficiency anemia is present after a person has been diagnosed with atrophic gastritis.