What is vitamin B12 called in blood tests?

Blood testing is central to diagnosing and managing vitamin B12 status. Clinically, the most common label you will encounter is serum B12, which reports the total concentration of cobalamins circulating in the blood. However, several other names and markers are used to give a fuller picture of B12 availability and function.

Key names and markers

Here are the terms you are most likely to see on laboratory reports:

• Serum B12: Total vitamin B12 measured in blood (often pg/mL). This is the usual first-line screening test.
• Holotranscobalamin (holoTC) or active B12: The fraction of B12 bound to transcobalamin II that is available to cells; often reflects bioavailable forms such as methylcobalamin.
• Methylcobalamin / Adenosylcobalamin: Biologically active forms of B12 that participate directly in metabolic reactions; some specialized assays or LC-MS methods can distinguish these forms.
• Methylmalonic acid (MMA): A functional marker that rises when intracellular B12-dependent metabolism is impaired.
• Homocysteine: Another metabolic marker elevated when B12 (or folate/B6) dependent pathways are compromised.
• Cyanocobalamin: The synthetic form used in many supplements and fortification; it contributes to total serum B12 but requires conversion to active forms.

Why multiple tests matter

A normal serum B12 does not always guarantee adequate cellular B12. In some cases, total serum B12 may include inactive B12 bound to haptocorrin, masking a functional deficiency. Measuring holoTC or active B12 (often reported with reference ranges in pmol/L) and checking MMA or homocysteine helps distinguish true deficiency from misleading blood concentrations.

How tests are done

Laboratories use different analytical platforms—immunoassays for serum B12 and holoTC, and mass spectrometry for form-specific measurement or MMA. The choice depends on clinical context: screening versus evaluating unexplained neurological symptoms or monitoring therapy.

Practical considerations

Supplement intake shortly before blood draw, especially cyanocobalamin, can transiently raise serum B12 and obscure deficiency. Clinicians often interpret results in light of symptoms, dietary history (e.g., vegan diets), age, and conditions affecting absorption. For an accessible overview aimed at general readers, see this targeted article on how B12 is reported in lab testing: What is vitamin B12 called in blood tests?.

For broader context on nutrient-focused wellbeing and related topics, the site home is useful as a reference: Topvitamine. If you are exploring complementary health topics, an article on adaptogens provides background on stress and cognitive support: unlocking the power of adaptogens for stress relief. A concise external summary of adaptogens is also available on Telegraph: Adaptogens summary on Telegraph.

Summary

In short, "serum B12" is the typical label on blood tests, while terms like holotranscobalamin, methylcobalamin, MMA and homocysteine provide additional and often more clinically informative detail. Combining these markers with clinical assessment gives the best chance of detecting true deficiency and guiding appropriate management.