A vitamin B12 level test checks the amount of vitamin B12 in the body. Normal levels usually range from 200 to 900 picograms per milliliter. The test can be done through a simple blood test. Low B-12 levels can lead to symptoms including fatigue, weakness, and anemia. A serum cobalamin level less than 200 nanogram/litre (ng/L) is appropriate in most cases to diagnose deficiency.