I wrote some time ago to say that following having lots of symptoms of B12 problems for some time I paid for an online extended B12 test which showed I was positive for intrinsic factor antibodies but normal B12. I gave these results to my GP who arranged their own NHS intrinsic factor test which has come back negative. However the range that the NHS test used was 0.0 - 25! Everything i have ever read says that a result over 1.53 AU is positive and mine was 3 AU . Does anyone know what unit of measurement they are using to have such a different range? Sorry if I've missed something here but I just want to make sure I'm not missing something or read it wrong. It does say it changed in 2015 but I can't find why.Attached is the screen shot from the NHS app.
Thanks so much clever people!
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Tinglypalps
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its an 'arbitrary unit' and really just a measure of how the machine has been calibrated, because there isn't an obvious unit for the test.The results give the normal range as 0-25 AU so a positive would be a result outside of this range.
I am guessing your battle with your GP is going to be to get him to recognise that this result doesn't rule out PA as the cause of a B12 deficiency, where as having a prior positive is good evidence that you do.
Refer them to the BCSH guidelines on diagnosis and treatment of cobalamin disorders which has a section that discusses the strengths and limitations of various tests, including IFAB in detail
IFAb is a major issue when it comes to detection. There are numerous ways of detecting the so-called 'intrinsic factor antibody-like effect' which is a reaction; inhibiting, binding or blocking intrinsic factor, but we don't have a 'gold standard' for this test. It's complicated; very complicated, and then, it's probably even more complicated than that.
Issuing results using 'AU' is likely to be misleading, and not every lab performing the test will be using the same 'AU' either, so it makes a fairly arbitrary test look rather more scientific than it actually is. There are no doubt perfectly reasonable arguments for doing this, but it doesn't make it helpful to the 'consumer'.
Gambit's observation sums it up nicely. There isn't a unit for the test.
It's a binary situation; there/not there. Positive/Negative. Then there are the few in the middle; weak reacting samples that are better repeated in a few weeks.
Ah I see, thanks both! I guess the NHS negative will trump the private positive so back to the drawing board of getting a proper diagnosis for whatever is going on!
If you click on "project documents" then on "consultation comments and responses" it gives a good insight into current issues around diagnosis and treatment.
Track down the local guidelines on treatment/diagnosis of B12 deficiency for your area of UK and see what it says about IFA tests and PA diagnosis and Antibody Negative PA.
Search forum posts for "local guidelines" or submit a FOI request to ICB (Integrated Care Board) / Health Board for your area asking for a copy of or link to local B12 deficiency guidelines.
Keep an eye on local guidelines, they are likely to change as CCGs (Clinical Commissioning Boards) have been replaced this year by ICBs (Integrated Care Boards). See my recent thread on Integrated Care Systems.
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