Vitamin B12 (cobalamin; B12) is an essential micronutrient, but deficiency is common. The prompt diagnosis and treatment of B12 deficiency protects against megaloblastic anaemia, neuropathy and neuropsychiatric changes. Biomarkers of B12 status include the measurement of serum B12 (also known as total B12 or serum cobalamin), holotranscobalamin (holoTC or 'active B12'), methylmalonic acid (MMA) and total plasma homocysteine (Hcy). There is no 'gold standard' test for deficiency and the sensitivity and specificity of each biomarker for the evaluation of B12 status is affected by analytical and biological factors that may confer a high degree of diagnostic uncertainty. Limited access to technical and clinical expertise can lead to an over-reliance on the serum B12 test, which is readily available and highly automated. In some cases, the sequential use of different B12 status biomarkers or the calculation of a composite B12 status score, derived from a panel of B12 biomarkers and adjusted for folate status and age, can be used to detect deficient states that may otherwise be overlooked when using a single biomarker approach. This review summarizes the utility of B12-related biomarkers and describes approaches to their application and interpretation.
Keywords: NICE; Vitamin B12; active B12; homocysteine; methylmalonic acid; serum B12.