Pernicious Anaemia Society

B12 and Active B12

I've just recently joined the Thyroid community -- I have Hashimoto's and Coeliac -- and was asked to post this query here... Checking back through my B12 tests I can see that I am skipping along the lower 10-20% of the range for B12, and the top 10-20% range for Active B12. I'm in Sydney and testing for Active B12 is standard if B12 is low; see What is the situation in UK, Canada etc and how are the tests different, and how reliable?

3 Replies

In the UK most doctors have never heard of an active B12 test and few are keen to order one.

The serum B12 test is notoriously unreliable. It has a large random error (so a true value of 200 might give test results of 160 - 240) and it can suffer from large systematic errors (high levels of anti-IF antibodies, as found in people with PA, can give false-normal or high results -

The 'normal' range is set so that it catches most people with a deficiency (not all) without flagging to many normal people as deficient.

The serum test is neither accurate or precise

The active B12 test is both more accurate and more precise.


active B12 has one advantage over serum - that it only looks at the forms of B12 that are bound to proteins that allow them to pass into cells.

However, like serum B12 it is still a measure of what is happening in the blood and not what is happening at the cell level - there are different indirect tests that can help with that namely MMA and homocysteine

Blue_Bee - your results are still within the normal ranges on both even if towards top on one and bottom on the other. 5% of people who are off the bottom of the range will actually be okay.

overlap of symptoms with thyroid problems is going to make evaluating symptoms difficult - so MMA might help


Wow, this is really helpful. Thank you! Fx


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