Did he/she say why they need to know if there is a deficiency? What is the documentation for, other than if the doctor thinks you are taking something you don't need to be taking? And what if a simple blood test appears to show that you don't have a deficiency, even though it makes you feel better?
Ultimately, if you need less of what has helped you, that will become obvious over time (you'll forget a dose and realize you didn't notice, etc.), and if the dosage you are on is a problem, things will get worse for you, or you may have noticed a worsening already.
I've been taking B-12 for several years but there were times at the beginning when I stopped because of the start-up symptoms, and it was pretty awful. I did have a B-12 deficiency, and it was helpful to know that, but a lot of people on here are really helped by methylation without having a severe deficiency. These "deficiencies" may not show up on lab tests, which is why many on here do the genetic testing. I found that when I stopped having gluten in my diet, I got a pretty severe canker sore, which is an indication of a folate deficiency, but my labs showed excessively high serum folate. But my genetic tests showed I have a problem with folate, and indeed, nearly 1000 mcg of folic acid per day via my supplements, was not working for me -- I needed methylfolate because I could not adequately convert the folic acid to a useful form.
So, ultimately, lab tests may not be the answer. Also, you may have to have multiple tests, since everyone is different. Maybe you do it once and it comes out just above a limit. Should you wait and do it in another month? Or assume you don't have a deficiency? What happens when the doctor says, well, I don't think you really do have a deficiency.
If its an issue of possible folate masking a B-12 deficiency, you can just increase the B-12 over time to 2,000 mcg per day or so, and you should be covered. Most doctors have tried to limit my B-12 even though I had a severe documented deficiency, which shows how little they know, sadly. One doctor recently admitted this to me, and recommended a book (forgetting the title) about a family who adopts a little girl from China and when she comes to the US, a lot of physical problems crop up, so they run test after test after test -- it turns out she was simply deficient in several key vitamins because of the awful orphanage she was in, but the doctors had no understanding of how to even recognize that. I was amazed she admitted it to me, as she typed non-stop during the whole 5 minute visit, but it was a rare glimpse of that bizarre world they work in.
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