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Pernicious Anaemia Society
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B12 test. Should I have fasted?

Had a blood test last week. One of the tests was for B12. This was added because I’m experiencing constant pins and needles. The number came back as 221 which was in the normal range of 200-900 listed on the results printout. I wasn’t told to fast for this test and so had eaten breakfast a couple of hours beforehand. I have since discovered (by googling) that I should have fasted for 6-8 hours.

So my question is ‘did not fasting skew my result’ . Would my result have been lower If I had?

13 Replies

Hi Andrewcr was the test solely for Vitamin B12?

I can understand fasting for an HbA1c test (or maybe even Folate - B9) but doubt it was necessary for B12 as it would take a very long time travelling through the whole of your digestive system before it got into your bloodstream from food.

Your level is bumping along the bottom of the range. Is your doctor not suggesting any treatment?

What sort of diet do you have?

Was your Folate level tested?

I am not a medically trained person but I've had Pernicious Anaemia (a form of B12 deficiency) for more than 46 years.

I wish you well


Hi Clivealive,

Will check to see if they tested folate. The test wad for a full blood count, b12 and a number of other things.

My diet is pretty healthy and not vegetarian. I do suffer from GERD and take omeprazole and have been for 3.5 years. Dr is not suggesting anything.


Well the Omeprazole will have an adverse effect on absorption of Vitamin B12 which relies on high stomach acid levels.

Symptoms of B12 deficiency tend to develop slowly and may not be recognised immediately. As the condition worsens, common symptoms include:

Weakness and fatigue

Light-headedness and dizziness

Palpitations and rapid heartbeat

Shortness of breath

A sore tongue that has a red, beefy appearance

Nausea or poor appetite

Weight loss


Yellowish tinge to the skin and eyes

If low levels of B12 remain for a long time, the condition also can lead to irreversible damage to nerve cells, which can cause the following symptoms:

Numbness and tingling in the hands and feet

Difficulty walking

Muscle weakness


Memory loss




The British Society for Haematology guidelines say on the Diagnosis of B12 and Folate Deficiency "In the presence of discordance between the test result and strong clinical features of deficiency, treatment should not be delayed to avoid neurological impairment".


Make a list of your symptoms and present this to your doctor and ask him to treat you according to your symptoms and (perhaps) even start you on loading doses "until there is no further improvement" according to the N.I.C.E guidelines below. Click on the link, then on "Scenario: Management" and scroll down to "Treatment for B12 deficiency"


If possible take someone with you who can validate your neurological symptoms as the doctor is less likely to pooh pooh you in front of a witness.

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What is the chance of recovery from nerve damage after starting the replacement if absorption issues are ruled out. Is the nerve damage permanent and what should be the correct treatment.


Hi Dvpk24

It is not uncommon for some symptoms to appear to get worse before they get better as the B12 you are having starts repairing the damage done to your nervous system and your brain starts getting multiple messages from part of the body it had "forgotten about" or lost contact with.

I sometimes liken it to a badly tuned radio on which you have turned the volume up high trying to catch the programme you want when all of a sudden the signal comes in loud and clear and the blast nearly deafens you.

A lot will depend on the severity and longevity of your B12 deficiency as to how long before there is no further improvement or recovery.

Some symptoms will "disappear" quite quickly whereas others may take months or even years. There is no set timescale as we are all different.

Unfortunately, a severe deficiency may cause damage to brain and nerve cells that is very difficult to reverse and early detection can be critical in preventing permanent neurological damage.

This last paragraph is not meant to scare you but it is the "worst case scenario" if the deficiency is not treated effectively as the P in P.A. is rightly called pernicious....


I was diagnosed at levels that are undetectable back in sep 17 (less than 146 in US) by the machine per my doctor as thats their lowest set range. I started to have symptoms from the beginning of the 2017 with severe headaches , depression, anxiety, electric shocks fatigue etc.. Does a Brain MRI rule out the damage to Brain. I had a Brain mri just before the B12 diagnosis and spine mri after the diagnosis which are almost normal except the spine showed some minimal degeneration . I was under treated for five months just with one shot per month for five months and then i started my own treatment by by taking high does methylcobalamin sub linguals and weekly shots and now daily injections from last month. I had some symptoms lessened but i have some weakness in left leg and some gait problems and anxiety which got worse. Do i need to ask for another MRI . Can brain damage happen in few months that can be visible on a MRI. I am functioning better with my brain fog reduced headaches and concentration increased but some gait problem which started in January is kind of improving slowly sometime and sometimes get worse i feel. Back and leg pain is worse though atm. All my problems started after a year of dental procedures with Nitrous oxide and before that i was healthy .TIA...


You've certainly been "through the wars" but hopefully your improvements will continue. Do you know what your Folate level is as this is needed to help process the B12 you are injecting.

I was 13 years after gastric surgery for the removal of two thirds of my stomach in 1959 before I (eventually) got the P.A. diagnosis at which time my doctor gave me (a 30 year old with a sick wife and two children under six) two years to live - unless I ate raw liver three time a day or had four weekly cyanocobamalin B12 injections for the rest of my life.

My ongoing symptoms are mainly numbness in my thighs, unsteady gait (my brain never seems to know where my feet are) occasional memory lapses and a return of some neuropathy and exhaustion in the run up to my next injection which I now have every three weeks.

Sadly that lovely doctor who cared for my whole family (she even played the organ at a daughter's wedding) died last year at the age of 92 but I'm still "clivealive" and now 77.

It's my bedtime now so I'll bid you goodnight from the U.K.

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Yes I am still going through. Can I ask if you have suffered with symptoms for 13 years before getting a diagnosis or when did you start to have symptoms before diagnosis. Am I correct in saying you had a nitrous oxide anesthesia during your surgery which caused PA ? My folate when diagnosed was 17.


I was only 17 years old when my peptic ulcer burst and had the stomach surgery and can only assume that the anaesthetic was nitrous oxide but if I had any stores of B12 in my liver that "survived" I must have lived off them for nine years.

I say nine years because in 1958 my doctor sent me for a "Schilling's" test which back in those days was what was used for checking for P.A.. It involved being given a massive dose of B12 by injection plus a glass full of radioactive B12 which I had to drink and the amount that was excreted in my urine was measured. Unfortunately the test was not much more reliable than the present day one and is no longer used because of the difficulty in obtaining radioactive isotopes needed.

The result came back "inconclusive". By this time I was on various medications including valium, librium, tofranil (for hypertension and depression) and neutradonna (an antispasmodic containing belladonna) for my heaving stomach.

However it would appear that the massive dose of B12 (and remember I'm not medically trained) enabled me to get off the medications (apart from tuinal - a sleeping pill) get sent by the then Ministry of Labour on a five month retraining course in book-keeping and accountancy enabling me to get back into work in February 1970 after 3 years off sick - an employment with the same firm I held for the next 27 years - and I was then registered as disabled due to my inability to carry on working in my previous employment as a qualified electrician because of the stomach surgery.

The accounts course was held in London and I was just one of 90+ other "disabled" persons many of whom displayed lost limbs or other physical losses whereas I stood six feet four with apparently "nothing wrong with me". It was held on the third floor (no lift) in an old school building and we were split into three groups. We learned book-keeping, accounts, commerce, English, Maths and manual typewriting. No two lessons were in the same classroom and when we queried this and why we had to climb all those stairs were told "If you can cope with this now you will manage in an office environment"

I survived the course, left on Friday 13th February 1970 and started work the following Monday, my Disablement Resettlement Officer (DRO) having managed to get me an interview with a firm in my home town 40 miles away from London where I was lodging for the course.

The B12 boost from two years before, a determination to succeed at the course and the need to get back into employment had kept me "driven" but by May 1972 I was beginning to flag again with a returns of the depression and Zombie like behaviour, the brain fogs and not knowing where my feet were etc and my doctor played a hunch and sent me for a second, unheard of Schilling's test which proved positive for P.A.

For the next nearly 40 years I lived in total ignorance of what P.A, was all about as I knew no-one else with it and none of the succession of nurses who gave me my every four weeks injections nor doctors whom I saw for other reasons ever asked my how I was "getting on". I began to notice a return of some neurological symptoms in the run up to my next injection so used to "slip in" a three weekly one now and again until the nurse "caught me out", refused to give me it and reported me to my then doctor who called me in like I was a naughty schoolboy. I tried to explain how I "knew" when it was the fourth week and my (now) wife confirmed that she had noticed that I often looked pale and became more irritable in the week before the next scheduled jab but he categorically refused to accept that it was the P.A. causing the symptoms and also refused to vary the prescription from "every four weeks".

So I joined the P.A. Society and asked on their then forum "am I the only one in the world......?" and was amazed at the response - NO! I was not alone.

I changed from that "one size fits all" doctor to another in the practice (I have been a patient at for 52 years since 1966) and I now have my injections every three weeks and supplement with a B12 spray in between times.

There is still a lot of ignorance about P.A. here in the U.K. and ignorance is not bliss.


Thanks for sharing the info and giving some confidence


clivealive HbA1c is a long term measure of blood sugars - no need to fast for it - you do need to fast for blood glucose test


You are so right Gambit62 and being diabetic I should have differentiated between the two tests but my mind didn't click.

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Fasting makes no difference. If the mechanisms by which you absorb B12 are functioning properly then your body maintains the levels in your blood so timing of food is not that relevant.

Point out to your GP that the test isn't that accurate - measurements on the same sample will result in results in a range of + or -20% of the actual result so you could well be deficient on that basis.

It is also a test of the amount that is in your blood not how well it is being utilised by your cells, which is what counts.

Used as a single measure it will miss 25% of people who are B12 deficient.

Also, 25% of people who are B12 deficient present without macrocytosis.

Source of this information is the BCSH standards on diagnosis and treatment of cobalamin and folate disorders. Your GP can access through the BNF


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