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, d 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.