Pernicious Anaemia Society
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Things that can affect a blood test?

It's rather a long story, so to keep it brief, I had a B12 level of 127 (reference range 180-900) last year, was very symptomatic, and have spent 12 months battling with GPs and haematologists to get treated properly. 

My last B12 injection was 29th February 2016 and I am due some blood tests on 7th April (which will be 5 and a half weeks since that last injection). My GP has agreed not to test my B12 levels as we both know they will be high (and I didn't want to give him any reason to stop injections altogether!). But I have asked him to do full blood count, iron levels (as I have suffered from iron deficiency anaemia in the past), potassium, calcium, liver work, thyroid and folate, as some of my symptoms have been returning and I want to rule out any other issues before tackling the "can I have more frequent B12 injections" battle.

Other than taking vitamin supplements, what are the other factors that can affect blood tests? For example, do iron levels vary throughout the month (for women!)? Can what you eat the days before a blood test affect any of the results? Having a glass of wine the night before? Does the time of day affect anything? Fasting? Are all these levels relatively stable or can they fluctuate significantly? etc ...

I obviously want to do my best to make sure the blood test is as accurate as I can, and don't want to inadvertently do anything that skews it!

Thanks for any advice :-)

5 Replies

Sorry not able to answer your question about what affects the blood test - except for the thyroid.  If you are looking for a diagnosis - the TSH is highest in the morning - so for a diagnostic result it is best to fast and only drink water and have the test as early as possible.  If they only test the TSH - then that is only part of the story - I would insist on the FT4 - FT3 and the anti-bodies - Anti-TPO and Anti-Tg.  Again most GP's have little understanding of the results - happy to help.

If your GP/Lab do not do the correct Profile for the Thyroid I would suggest you have them done privately through Blue Horizon via the main Thyroid UK website.....

B12 deficiency and low thyroid seemingly go hand in hand - chicken or egg ?

I am not a medic just a Hashimotos sufferer with B12 Deficiency :-)


Ask for a vitamin D blood test as well



Hope these links might be helpful.

Are you on any medication? Some drugs can affect levels of B12 and/or folate in blood.

See section on drug induced causes. I have also read that some antiepileptic drugs may affect b12 and folate levels.

Have you had any surgery involving nitrous oxide recently?

 "iron levels (as I have suffered from iron deficiency anaemia in the past)"

Have you had a full iron panel of tests not just a ferritin test?


Thank you all for the replies. I was also vit D deficient last year, and the Dr gave me tablets for 3 months. I did ask for that to be added to the blood tests this time round, but my GP basically said no, as it would be fine after taking the supplements. I didn't want to push too much as I am trying to keep him on my side.

Sleepybunny - thank you for the useful links. I have asked the dr for the full suite of iron tests so fingers crossed. I don't take any medication and haven't had any surgery, which is why I wanted to check whether there were any other day to day things that can affect test results.

Thanks again for all the help.


Your "Glass of wine" question triggered this response.  Although alcohol doesn't interact directly with B12, it does eliminate many of the other B vitamins in the body that B12 needs to do its job. For me, a drink is like 10. The hang over feeling lasts for days instead of hours. 

Do you keep a logbook of all your food and drink?   You may find you have new food intolerances like gluten and dairy (casein protein).  The logbook also serves as a reference when discussing the progress of your treatment with your GP.  It comes in handy when trying to get him to increase your B12 injections.  It becomes "evidence" that certain things are working and others are not.

It also helps with short term memory loss and looking back to see the very slow progress of any neurological repair.

Try to assess a severity scoring system for all your symptoms.  Some will only occur right after (within 48 hours) your B12 injection. These, although similar to other ones, are good indicators that B12 is working.  

When moving from testing to treating symptoms your logbook provides the tool to understand each symptom.

There is a 3 to 48 hour delay because the gut is still involved.  I get a soft poo about 12 hours after my jab.  This is normal because the metabolism kicks into high gear and byproducts such as water and urea flood the system.  The liver sorts most of this out which ends up in the gut.  The kidneys do the rest.

For me urine color is another key indicator of good B12 levels.


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