Health care providers may administer tests to measure the level of Vitamin B12 in your system for a variety of reasons. The most common is to help diagnose the cause of megaloblastic anemia, according to a report in the New York Times. Poor B12 absorption can cause pernicious anemia, a form of megaloblastic anemia. Other physical symptoms that may indicate B12 testing include weakness, dizziness, fatigue, a sore mouth or a sore tongue. Doctors also may order B12 tests to assist in diagnosing the underlying causes of behavioral changes such as confusion, irritability, depression, or paranoia, the Times notes.
Do not eat or drink six to eight hours before your test. Tell your doctor or health care provider about all supplements and medicines you are taking, advises the National Institutes of Health. Especially make sure to mention para-aminosalicylic acid, colchicine, neomycin or phenytoin (also known as Dilantin), as these potentially will affect your test results. Inform your doctor if you've been taking folic acid as this can mask a B12 deficiency.
Expect a slight pain, or stinging sensation, when your health care provider inserts a needle into one of your veins to draw blood for the test. The draw usually is done on the back of the hand or from inside your arm after the site is cleansed with antiseptic, according to the New York Times.
Discuss your results with your doctor. The National Institutes of Health advises that a normal B12 value is 200 to 900 picograms per milliliter. If your test indicates less than 200 pg/mL, you have a B12 deficiency.
Schedule further tests if your B12 test indicates a deficiency. A Schilling test, which involves administering radioactive B12, or tests that measure parietal cell antibodies and intrinsic factor binding antibodies may help determine the cause of your deficiency. Possible reasons for the deficiency may include not consuming enough of the vitamin, disorders that cause malabsorption such as Crohn's disease or celiac disease, hyperthyroidism, or pregnancy, according to the NIH. Use of oral contraceptives, alcohol, estrogens and some antibiotics also can lead to deficiency, according to Lab Tests Online.