Serum vitamin B12

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]