Vitamin B12 (cobalamin) is a water-soluble vitamin that plays a central role in nerve function, red blood cell production, and DNA synthesis. It is found almost exclusively in animal-derived foods including meat, fish, dairy, and eggs, which makes people following vegetarian and vegan diets particularly susceptible to deficiency.
Unlike most water-soluble vitamins, B12 is stored in the liver in relatively large amounts. This means deficiency develops slowly, often over months to years, and symptoms can be subtle and non-specific before they become clinically obvious. Common early signs include fatigue, weakness, poor concentration, and mood changes. More advanced deficiency can cause peripheral neuropathy (tingling and numbness in hands and feet), cognitive difficulties, and megaloblastic anaemia.
This test measures total serum B12, which includes both the active (holotranscobalamin) and inactive fractions circulating in the blood. It is the standard first-line test for assessing B12 status. However, total serum B12 has a recognised limitation: levels can appear normal even when functional B12 deficiency is present, because a large portion of circulating B12 may be bound to proteins that cells cannot use. For a more sensitive measure of early or borderline deficiency, the Active Vitamin B12 (HoloTC) test is available separately.
Groups at higher risk of B12 deficiency include vegans and vegetarians, people with pernicious anaemia (an autoimmune condition that impairs B12 absorption), those with Crohn's disease, coeliac disease, or other gastrointestinal conditions, individuals who have had gastric surgery, older adults (absorption declines with age), and people taking metformin or proton pump inhibitors long-term.
Folate and B12 work together in several metabolic pathways, and deficiency in either can produce similar symptoms. Testing both at the same time is often recommended to get a clearer picture.
To measure total serum B12 and identify levels that may be too low, particularly in people at risk of deficiency due to diet, absorption issues, or medication use.
