Active B12

Some laboratories use 37.5 to 188 pmol/L, and many quote a grey zone between about 25 and 70 where methylmalonic acid is checked next; use the range your lab printed.

Active B12, or holotranscobalamin, is the fraction of vitamin B12 that is bound to the carrier protein that actually delivers it into cells. It is about a fifth to a third of the total B12. Because it measures only the usable part, it gives a better guide to whether the cells are getting enough than the total B12 does, and it falls earlier when supplies are short.

Doctors, and several private testing services, request it instead of or alongside total B12 when looking into tiredness, nerve symptoms or large red cells, particularly when the total B12 sits in the lower part of its range and the answer is unclear.

• B12 injections or supplements
• Liver disease
• Long-term kidney disease
• Some blood conditions

• A diet with little animal food
• Pernicious anaemia and other causes of poor absorption
• Stomach or bowel surgery and conditions of the small intestine
• Metformin and acid-suppressing medicines, over years
• Older age

• A value in the grey zone is common in the general population and is usually followed by a methylmalonic acid rather than acted on alone
• A recent supplement raises it
• It is less affected than total B12 by pregnancy and the pill, but not entirely
• Different laboratory methods give somewhat different numbers

No fasting needed. Best taken before any supplements are started.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.