Vitamin B12 is an important water soluble vitamin responsible for correct blood production and a well-functioning nervous system. It is found primarily in meat and shellfish, meaning that deficiencies are frequently a problem for vegetarians, vegans and other people with restricted diets. Too little vitamin B12 in the bloodstream can produce pernicious anemia, neurological impairments like burning or numbness in the extremities and psychological symptoms such as depression and poor memory. Low levels of this vitamin can also be associated with early onset of Alzheimer's disease.

A blood test may show low levels of vitamin B12 if a patient doesn't consume an adequate diet, but this is not the only cause of deficiency. People who drink regularly or suffer from liver disease, patients with gastric cancer and people suffering from Chron disease, celiac disease, inflammatory bowel syndrome cystic fibrosis may have trouble absorbing the vitamin even when they eat correctly. Routine testing can help identify these conditions or help sufferers avoid the risk of more serious problems later.

It's important to FAST for 10 hours before the blood test. Patients who are taking chochicine, para-aminosalicylic acid, Dilantin or neomycin may show inaccurate results on a vitamin B12 test. Ideally, the results should show values of 200 to 900 picograms per milliliter, though results may vary slightly from lab to lab. When a vitamin B12 test shows a deficiency, patients can correct the problem by consuming shellfish, liver or vitamin supplements.