I'd like to suggest an alternative strategy for some of you to get diagnosed and treated for PA based on tests for one of the other "fringe benefits" of this dreadful disorder that many of you probably didn't even realize you have: gastritis accompanied by IFA (intrinsic factor antibodies).
If you have frequent attacks of acid reflux, gas, cramping, nausea, constipation and/or diarrhea, find yourself a good gastroenterologist and request a test for helicobacter pylori infection. The link between pylori and PA is well known to any "gastro" guy. Not only that, they'll tell you that the presence of h. pylori in the gut increases your risk of developing stomach cancer later on (as if we didn't have enough to worry about from good old PA, right?)
All they need to see is the slightest trace of intrinsic factor Ab in your blood, and it's a clear sign to a "gastro" that PA isn't far behind.
This is a really useful idea. I got my PA diagnosis 20 years ago, but the last GP I saw claimed I could only possibly have PA if my serum B12 was low! I pointed out the strongly positive antibody results I'd had and all the other auto-immune disease I deal with, but she just looked blank. She was a young doctor, not long qualified, and evidently training standards are not what they were in British medical schools.
I have my next gastroenterology appointment in January, so this could be a way to get antibodies re-tested. Forgive my ignorance, but I'm not clear on whether self-treatment with methyl may drive anti-bodies down and skew the test results? I know a similar problem occurs in Hashimoto's.
Hillwoman, I did find this from University of Rochester, but couldn't find answers from other sites at this time. Hope it's helpful or may point you to more information, as it's a good question.
Injected vitamin B12 could affect results. If you have had an injection of the vitamin, your doctor will probably ask you to wait for up to two weeks before testing."
I mentioned to my doc that I drank contaminated hotel water while on holiday in Bali 10yrs ago and had a seriously upset stomach that took long to settle. He said it couldn't be the cause as it was so long ago. I have since read that H pylori remains until treated with antibiotics????
Re:" H pylori remains until treated with antibiotics", H P can be very difficult to get rid of, so testing if its actually gone is a good thing. Perhaps read up a bit more in this topic, see:
A tummy full of h.pylori isn't the only permanent souvenir people take home from the tropics. Tapeworms from raw or under-cooked fish (sushi, etc.) can reside unnoticed for decades in the gut. There are more examples, but I don't want to gross you out
"When inflammation affects the gastric corpus, parietal cells are inhibited, leading to reduced acid secretion. Continued inflammation results in loss of parietal cells, and the reduction in acid secretion becomes permanent. "
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atrophic gastritis pernicious anaemia 
omeprazole and Lansoprazole
Reactive Gastritis?
Various B12D/PA resources 
Stomach acid and PPI's 
