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] 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.[5]

Normal range

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.[6] 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.[7] 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.[8]

References

  1. 1 2 3 4 5 6 7 Vollbracht, C; McGregor, G P; Kraft, K (2019-06-28). "Supraphysiological vitamin B12 serum concentrations without supplementation: the pitfalls of interpretation". QJM: An International Journal of Medicine. 113 (9): 619–620. doi:10.1093/qjmed/hcz164. ISSN 1460-2725. PMC 7550708. PMID 31251363.
  2. Sugihara, Takaaki; Koda, Masahiko; Okamoto, Toshiaki; Miyoshi, Kenichi; Matono, Tomomitsu; Oyama, Kenji; Hosho, Keiko; Okano, Jun-ichi; Isomoto, Hajime; Murawaki, Yoshikazu (2017-03-09). "Falsely Elevated Serum Vitamin B12 Levels Were Associated with the Severity and Prognosis of Chronic Viral Liver Disease". Yonago Acta Medica. 60 (1): 31–39. ISSN 0513-5710. PMC 5355842. PMID 28331419.
  3. 1 2 3 4 5 6 "Office of Dietary Supplements - Vitamin B12". ods.od.nih.gov. Retrieved 2023-10-14.
  4. 1 2 Herbert, V.; Colman, N.; Palat, D.; Manusselis, C.; Drivas, G.; Block, E.; Akerkar, A.; Weaver, D.; Frenkel, E. (November 1984). "Is there a "gold standard" for human serum vitamin B12 assay?". The Journal of Laboratory and Clinical Medicine. 104 (5): 829–841. ISSN 0022-2143. PMID 6387014.
  5. "Diagnosis and treatment pitfalls – B12 institute". Retrieved 2023-10-14.
  6. "Vitamin B12 or folate deficiency anaemia - Diagnosis". nhs.uk. 2017-10-20. Retrieved 2023-10-14.
  7. Snow, C. F. (1999-06-28). "Laboratory diagnosis of vitamin B12 and folate deficiency: a guide for the primary care physician". Archives of Internal Medicine. 159 (12): 1289–1298. doi:10.1001/archinte.159.12.1289. ISSN 0003-9926. PMID 10386505.
  8. Goodman, Mark; Chen, X. Helen (October 1996). "ARE U.S. LOWER NORMAL B12 LIMITS TOO LOW?". Journal of the American Geriatrics Society. 44 (10): 1274–1275. doi:10.1111/j.1532-5415.1996.tb01389.x. PMID 8856015. S2CID 34014860.
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