hi Riven, my ds B12 was OK, that was tested by the same neuro who did the gluten gene test. I looked up biotin AND it is one of the B vits (B7)! B12 is made by gut bacteria, so often probiotics can help.
We do tests with help from our paed, I managed to establish an excellent relationship with him over the years. I had told him in tears on one of the early occasions that I was not willing to give up even if everyone else had and desperately needed his help as maybe there was something we could do. A few years later when I came across gluten research, I started looking into it in depth.
The urine test we have had is called Urine Amino Acid screen. There could well be other useful urine tests too, I am still looking into it. I am also looking into a test for pyroluria, which would pick up issues with B6 & Zn deficiencies/borderline results (pyroluria can be also linked with some types of epilepsy).
Now, as and when I find out about anything relevant and worth checking, I write to our paed and ask for the tests. GPs have changed so much here that I wouldn't have hope or time to explain what and why. We still go to the hosp in London but we also see our paed 6-12 monthly for a catch-up chat. For us he is a lifeline where I know he won't fob me off. My ds had some blood tests in the past too, but I am now happy with a maintenance approach and address issues as and when.
Unfortunately, with most tests, we never get any commentary, more often than not it just says "no significant abnormality", so I am just left researching it line by line myself. Quite often if one or two substances were deficient, the overall result was still called "normal" and only when I worked out significance of them I got going on what could be done - and it wasn't at all "normal".
Like with histidine deficiency, this appears to have some effect on hearing and blow me down, my ds does have a slight high-frequency hearing loss, I am hoping that by fixing Zn he might at least stabilise; Zn acts synergistically with histidine and I think with taurine too, it's all like a big jigsaw puzzle. Also omega 6 are good for improving histidine.
I read some great reports on keto, both diets are not that far apart to be honest (gf less precise), we also try to keep low on carbs and good fats are good for health anyway.