Why And How Should Vitamin B12 Be Measured?
Vitamin B12 - What is it?
Vitamin B12 (B12), which is more correctly known as cobalamin, plays a vital role in the formation of red blood cells and in the function of the brain and nervous system. It is one of eight B vitamins, the others being B1, B2, B3, B5, B6, B7 & B9 (the missing numbers are due to the compounds being declassified as vitamins), all of which are involved in cell metabolism. Vitamin B12 is involved in the metabolism of every cell of the human body, especially affecting DNA synthesis, fatty acid and amino acid metabolism.
What are the manifestations of Vitamin B12 deficiency?
Risk factors for B12 deficiency include restricted dietary intake of animal products, impaired gastric absorption, loss or inactivity of intrinsic factor (Addisonian pernicious anaemia), pancreatic insufficiency, impaired intestinal absorption (e.g. ileal resection in Crohn disease), multiple congenital factors and acquired drug effects.
B12 deficiency is common, with a prevalence of ~5% in people 65–74 years of age and more than 10% in people 75 years of age or older.
The detection and correction of B12 deficiency prevents megaloblastic anaemia and potentially irreversible neuropathy and neuropsychiatric changes. Crucially, 20% of B12-deficient patients have no discernable haematological diathesis
Vitamin B12 - What are traditional diagnostic tests?
Testing for B12 status is problematic because no single laboratory marker is suitable for the assessment of B12 status in all patients. Most laboratories estimate B12 status by measuring the abundance of B12 in serum and comparison against a predefined reference range. However, this approach gives no indication of B12 utilisation and it is known that serum B12 assays generate a high rate of false-negative results. This means that up to 45% of B12-deficient subjects can be overlooked if only serum vitamin B12 assays are used as a screening test. The application of multiple laboratory markers greatly improves the diagnosis of B12-deficiency.
Novel markers of B12 status include holotranscobalamin (marketed as ‘active B12’). Holotranscobalamin is the form of B12 taken up by cells to meet metabolic demand. Laboratory B12 status markers that reflect cellular utilisation rather than abundance are also available. In humans, two forms of B12 act as coenzymes for two different reactions. Methionine synthase requires methylcobalamin for the remethylation of methionine from homocysteine. A homocysteine concentration >20 μmol/L may suggest B12 deficiency in folate-replete patients. In the second B12-dependent reaction, methylmalonyl-CoA mutase uses adenosylcobalamin to convert methylmalonyl-CoA to succinyl-CoA. In B12 deficiency excess methylmalonyl-CoA is hydrolysed to methylmalonic acid. A serum concentration >280 nmol/L may suggest suboptimal status in young patients with normal renal function.
Vitamin B12 - How is Vitamin B12 measured at Viapath?
The Viapath Nutristasis Unit has used holotranscobalamin as the first-line marker for the assessment of B12 status since 2012, a two-step immunoassay using chemiluminescent microparticle immunoassay (CMIA) technology. The Unit has found that ~5% of requests from mixed patient populations have a holotranscobalamin <25 pmol/L and are classified as deficient based on this test alone; an indeterminate result of 25–70 pmol/L is measured in ~25% of samples leading to secondary test using methylmalonic acid analysis – of these one in three patients are found to be deficient. All other samples have a holotranscobalamin concentration >70 pmol/L and are classified as B12 replete.
If you would like further information then please refer to the recent Best Practice article:
Harrington DJ. J Clin Pathol 2016 http://jcp.bmj.com/content/early/2016/05/11/jclinpath-2015-203502.long
