Vitamin B12 testing measures the level of vitamin B12 in your blood. Having a vitamin B12 deficiency, or a low level of this essential vitamin, can cause a range of health issues, including anemia and nervous system problems. Vitamin B12 levels may be low for a number of reasons, including inadequate dietary intake, the use of certain medications, advanced age, or conditions that make it harder for the body to absorb vitamin B12 from your food. Testing for vitamin B12 requires a sample of blood. Measuring your vitamin B12 level helps your health care provider to diagnose a possible deficiency and recommend appropriate treatment, if needed.