Serum vitamin B12 is a medical laboratory test that measure vitamin B12 only in the blood binding to both transcobalamins.[1] Most of the time, 80–94% of vitamin B12 in the blood binds to haptocorrin, while only 6–20% is binds to transcobalamin ll.[2] Only transcobalamin ll is "active" and can be used by the body. Normal total body vitamin B12 is between 2 and 5 mg with 50% of that stored in the liver.[3] Total serum vitamin B12 may not be a reliable biomarker for reflecting what the body stores inside cells. Vitamin B12 levels can be falsely high or low and data for sensitivity and specificity vary widely. There is no gold standard human assay to confirm a vitamin B12 deficiency.[4]
Healthcare providers use this test when a vitamin B12 deficiency is suspected, which can cause anemia and irreversible nerve damage. The cutoff between normal vitamin B12 levels and deficiency varies by country and region. A diagnosis of vitamin B12 deficiency is determined by blood levels lower than 200 or 250 picograms per ml (148 or 185 picomoles per liter). Some people can have symptoms with their normal levels of the vitamin, or may have low levels despite having no symptoms.[5] Other tests may be done to ensure individuals status. Measuring vitamin B12 values in individuals during or after treatment, in order to measure the effectiveness of treatment, is useless.[6]
A blood test shows vitamin B12 levels in the blood. Vitamin B12 deficiency can be determined, but not always. This means it measures forms of vitamin B12 that are "active" and can be used by the body, as well as the "inactive" forms, which cannot.[7] However, also normal or supraphysiological vitamin B12 levels should be carefully assessed in the context of the individual state of health. Elevated or normal serum vitamin B12 levels may also be associated with a functional vitamin deficiency. Functional deficiency has been described despite high B12 concentrations and is due to a failure of cellular uptake, intracellular processing, trafficking, or utilization. However, low vitamin B12 levels may occur other than the true deficiency for various reasons and circumstances.[8] High or supraphysiological serum levels are usually not of concern, although without supplementation they have been associated with many pathological conditions.
Laboratories often use different units and "normal" may vary by population and the lab techniques used. Some researchers have suggested that current standards for vitamin B12 levels are too low.[9]