there are two treatment protocols in NICE and BCSH guidelines that apply in the UK
you are being treated according to the protocol where there isn't neurological involvement
in cases where there is neurological involvement protocol is loading doses 3x per week until symptoms stop improving followed by maintenance shots every two months
your GP is not currently treating you per guidelines and whilst they are supposed to use their professional judgement I suspect the actual reason is lack of awareness of the existence of the second protocol for neurological symptoms so I personally would advise asking why you are not currently being treated per guidelines before undertaking any further testing.
Iron and Vit D are not relevant to deciding whether you need more B12 - they are tests for other conditions (iron based anaemia) and Vit D deficiency that overlap with symptoms of B12 deficiency.
If you have been self injecting then tests in two weeks are unlikely to be informative.
If not then tests that might potentially be useful would be serum B12, MMA and possibly folate.
IFA is test for pernicious anaemia which would explain why you have become deficient but not give any clue as to what treatment suits you best.
IFA is also prone to false negatives 40-60% of the time depending on the assay method used - so it is a long way from proving that you don't have PA.
Serum B12 is a pretty blunt instrument for diagnosing B12 deficiency. However, normal range definitely can't be used as a guide to whether you are receiving the right frequency of injections after someone has received loading shots and is only significant as an indicator if it comes back low - which would show that you are removing B12 from your system quickly.
Serum B12 is a measure of how much B12 is in your blood - but there is a lot more to a deficiency than the amount in your blood and it is the amounts available in your cells that are important but there isn't a direct measure of this. High levels of serum B12 (inevitable if you have had an injection) can cause a reaction that stops the B12 getting from your blood to your cells unless the levels in your blood are very high and this is why high levels after treatment starts can't be used to show that you are getting the amounts of B12 you need. MMA can provide some guide as it will be raised if your cells don't have the B12 they need to recycle it. [However, I'm personally not convinced that MMA will be a definitive guide, but I'm not a biochemist - that's just based on it being an indirect indicator, people being people and there being a lot of genetics that affects personal biochemistry etc etc.]
there is no point in having the blood tests if you have been self injecting - particulary if you haven't discussed self injection with your GP.
The only test that would be worth doing - and you would need to stop supplementing for ten-fouteen days before hand to avoid the possibiltiy of a false positive - would be the IFA test
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