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.[1] 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.[1] 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.[3] The cutoff between normal vitamin B12 levels and deficiency varies by country and region.[3] 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).[3] 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.[3] 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.[3] Vitamin B12 deficiency can be determined, but not always.[4] 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.[1] Elevated or normal serum vitamin B12 levels may also be associated with a functional vitamin deficiency.[1] Functional deficiency has been described despite high B12 concentrations and is due to a failure of cellular uptake, intracellular processing, trafficking, or utilization.[1] 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.[1]
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]