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Pernicious Anaemia Society
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How long before B12 tablets won't show in blood test?

I am having a bit of a battle with my doctors trying to get diagnosed with B12 deficiency. I have been taking B12 tablets and seeing little change and am now considering paying for an active B12 test to take to my doctors as amo to persuade them to try me on the B12 injection. Does anyone know how long it takes before the tablets I have been taken won't show in my blood test result as I don't want them skewed?

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link about Active B12 test


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Sorry to say but it is possible it may be quite a few months if the tablets were high dose ones. Unfortunately how long kind of depends on how quickly you get rid of B12 and everyone is different. :-(

Active B12 test may not give you much more info than an ordinary Total B12 blood test - neither show how well B12 is getting into your cells and being used by the body. MMA or homocysteine would show if you are deficient biochemically though these can be affected by other medical issues like low folate and kidney problems as well as other things.

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Hi acdoor. I see that you're saying you're having trouble getting your doctor to diagnose you with B12 deficiency.

Most doctors don't understand that a serum B12 level can be within what is called the normal reference range, but you can still have B12 deficiency. And few understand that they should be treating symptoms, not just looking at the test results.

I'm sorry but I don't have time to leave a lengthy reply but I thought I'd leave you some links to information and guidance that will help you to understand what your GP should be doing for you. It will also give you evidence to take along and show your GP in support of your request for treatment.

Also note: your GP should test your folate levels. Folate and B12 work together so if you are deficient or low in folate your body will not be able to utilise 12 properly.

I agree with taka about the B12 supplements skewking test results. Your GP should take into account the fact that you have supplemented and recognise the impact this will have on your serum B12 levels, rather than just assuming that all is okay.

Here are the links to information and guidance. It looks like a lot of reading, but don't worry, most of the documents are only one page long πŸ˜„.

If you have any questions when you've finish reading or you need any more help with your GP...or just want a bit of support, please put up a new post. Lots of lovely folks here to pop along and help. πŸ‘

b12researchgroup.wordpress.... (Serious Caution Note about the use of Oral and Sublingual B12 Supplements)

evidence.nhs.uk/formulary/b... (BNF B12 Deficiency: Hydroxocobalamin Treatment Regimes)

pernicious-anaemia-society.... (PAS Symptom Checklist)

stichtingb12tekort.nl/weten... (BSH B12 Deficiency / PA Diagnostic Flowchart)(British Society Haematology (BSH) Guidelines: Treatment of B12 Deficiency and Folate Disorders)

cks.nice.org.uk/anaemia-b12... (NICE Guidelines Treatment of B12 / Folate Deficiencies)

onlinelibrary.wiley.com/doi... (UKNEQAS B12 Treatment Alert, Neurological Symptoms and Risk of Subacute Combined Degeneration of the Spinal Cord)

stichtingb12tekort.nl/weten... (Problems with Serum B12 Test)

stichtingb12tekort.nl/weten... (Testing B12 During Treatment)

stichtingb12tekort.nl/weten... (Misconceptions About B12 Deficiency – Good to Know Before Seeing GP)

stichtingb12tekort.nl/weten... (B12 Treatment Safety / Long Term Treatment for neurological symptoms)

stichtingb12tekort.nl/weten... (B12 Deficiency: Neurological Symptoms Can Present Even When B12 is β€˜In-Range’ and Without Macrocytosis (large red blood cells) or confirmed PA Diagnosis)t

stichtingb12tekort.nl/weten... (B12 and Intrinsic Factor)



Thank you all for your replies. I shall have a read of all that stuff


Isn't this forum brilliant? So many lovely, helpful people! It's helped me get through a very frightening period of my life when I seriously thought I may have something terminal, and reassured me on a daily basis that I'm doing the right thing. Lovely PAers!

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Havegot exact same problem.SWydochesaid15yearsagohe would ha would put me on it but nhs chAnged goal post.i have started using b12 patches it absorbed straight into blood stream..It company called living nuttritals found it on amozen then got to company through that.i only been using them for a week now so remains to be seen.It same dose as the injection.There is pressure on government if look it up they getting as many people as possible I have signed on line.

I guess you feel crappy like me

So good luck


Hi, I'm having same problem too! I started b12 injections and peals before seeing the doctor! Was a mistake on my part, I don't know about pa just thought it was a case of having extra b12, until my mom told me my nan nearly died from pa, so I thought I better go see the gp, (GP stands for general pratt) lol anyway I told her what I had taken, had 2000mcg of sublingual b12 on morning too! And loading on injections, she took my bloods regardless and hay ho my serum levels were normal, if you read one of the links somebody's has sent you about sublinguals it says ten days to two weeks, but as she said we are all different! Please let me know how you get on


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