Serum vitamin B<sub>12</sub> is a medical laboratory test that measure vitamin B<sub>12</sub> only in the blood binding to both transcobalamins. Most of the time, 80âÂÂ94% of vitamin B<sub>12</sub> in the blood binds to haptocorrin, while only 6âÂÂ20% is binds to transcobalamin ll. Only transcobalamin ll is "active" and can be used by the body. Normal total body vitamin B<sub>12</sub> is between 2 and 5 mg with 50% of that stored in the liver. Total serum vitamin B<sub>12</sub> may not be a reliable biomarker for reflecting what the body stores inside cells. Vitamin B<sub>12</sub> 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 B<sub>12</sub> deficiency.
Healthcare providers use this test when a vitamin B<sub>12</sub> deficiency is suspected, which can cause anemia and irreversible nerve damage. The cutoff between normal vitamin B<sub>12</sub> levels and deficiency varies by country and region. A diagnosis of vitamin B<sub>12</sub> 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. Other tests may be done to ensure individuals status. Measuring vitamin B<sub>12</sub> values in individuals during or after treatment, in order to measure the effectiveness of treatment, is useless.
A blood test shows vitamin B<sub>12</sub> levels in the blood. Vitamin B<sub>12</sub> deficiency can be determined, but not always. This means it measures forms of vitamin B<sub>12</sub> that are "active" and can be used by the body, as well as the "inactive" forms, which cannot. However, also normal or supraphysiological vitamin B<sub>12</sub> levels should be carefully assessed in the context of the individual state of health. Elevated or normal serum vitamin B<sub>12</sub> levels may also be associated with a functional vitamin deficiency. Functional deficiency has been described despite high B<sub>12</sub> concentrations and is due to a failure of cellular uptake, intracellular processing, trafficking, or utilization. However, low vitamin B<sub>12</sub> levels may occur other than the true deficiency for various reasons and circumstances. 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 B<sub>12</sub> levels are too low.