Serum vitamin B12

Serum vitamin B₁₂ is a medical laboratory test that measure vitamin B₁₂ only in the blood binding to both transcobalamins. Most of the time, 80–94% of vitamin B₁₂ in the blood binds to haptocorrin, while only 6–20% is binds to transcobalamin ll. 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. 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.

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. 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.