# What is vitamin B12 called in blood tests? ## Introduction Vitamin B12 is an essential micronutrient involved in DNA synthesis, red blood cell production, and nervous system function. Deficiency risk increases with age, restrictive diets, and malabsorption. Blood testing is the primary way clinicians and nutrition professionals assess B12 status to guide dietary or supplement decisions. ## Common blood tests for B12 Serum B12 is the standard screening test and reports total circulating vitamin B12. Results are easy to obtain but do not separate active and inactive forms. When symptoms or risk factors exist, clinicians often use additional tests to clarify status. ## Active B12 and methylcobalamin Active B12 is measured as holotranscobalamin (sometimes reported as methylcobalamin in research contexts), which reflects the fraction available to tissues. Measuring active B12 helps detect functional insufficiency when total serum B12 appears normal. Laboratories use immunoassays or more specific techniques like LC-MS for form-specific detection. ## Functional markers Methylmalonic acid (MMA) and homocysteine are metabolic markers that rise when B12-dependent reactions are impaired. Elevated MMA is more specific for B12 deficiency, while homocysteine can also reflect folate or B6 status. Combining serum B12, active B12, and functional markers gives a more complete diagnostic picture. ## Further reading and resources For an overview aimed at consumers, see this Topvitamine blog post on B12 blood tests: [What is vitamin B12 called in blood tests?](https://www.topvitamine.com/blogs/news/what-is-vitamin-b12-called-in-blood-tests). Additional practical guidance on identifying supplement quality is available here: [How to spot counterfeit supplements before you buy](https://hackmd.io/@topvitamins/how-to-spot-fake-or-counterfeit-supplements-before-you-buy). For context on complementary approaches, read [Unlocking the power of adaptogens for stress relief and mental clarity](https://hackmd.io/@topvitamins/unlocking-the-power-of-adaptogens-for-stress-relief-and-mental-clarity). A plain summary on the topic is also hosted at Telegraph: [How to Spot Fake or Counterfeit Supplements](https://telegra.ph/How-to-Spot-Fake-or-Counterfeit-Supplements-Before-You-Buy-12-25). ## Conclusion Knowing the terms used in blood tests - serum B12, holotranscobalamin, methylcobalamin, MMA, and homocysteine - helps interpret results and plan interventions. Test selection should align with clinical context and be discussed with a healthcare professional. Evidence-based measurement and follow-up improve the chances of identifying true deficiency and guiding safe supplementation.