The Vitamin B-12 test measures the level of vitamin B-12 (cobalamin) in the blood, a vital nutrient that plays a key role in maintaining healthy nerve function, red blood cell formation and DNA synthesis. This test is often recommended on experiencing symptoms like fatigue, weakness, memory issues, numbness or tingling in hands and feet and even mood disturbances; or with conditions like anemia or gastrointestinal disorders that affect nutrient absorption.