Serum vitamin B12

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Serum vitamin B12 is a blood test that shows how much B12 is circulating in the blood. In the blood, most B12 binds to a protein called haptocorrin, and only a smaller portion binds to transcobalamin II, which is the form the body's cells can use. The body stores about 2–5 mg of B12, with roughly half stored in the liver. A normal total B12 level does not always reflect how much B12 is stored in body cells, and test results can be falsely high or low. There is no gold standard test to prove B12 deficiency. Doctors order this test when a deficiency is suspected, because lack of B12 can cause anemia and nerve damage if untreated. Cutoff values for deficiency vary by country and lab; common thresholds are around 200–250 pg/mL (148–185 pmol/L). Some people have symptoms even with normal B12, or have low levels without symptoms. Other tests may be used to confirm status. Measuring B12 during or after treatment is not reliable for judging effectiveness. High or normal serum B12 levels can occur with functional deficiency, due to problems with cellular uptake or use. Labs use different units and reference ranges, and some researchers think the standards are too low.