A blood test for celiac disease rules out celiac (essentially allergy to Gluten), it doesn't rule out intolerance. You can definitely have problems with gluten even without having celiac disease. Some people can't tolerate any at all, some people can manage ancient grains but not wheat... None of my family are celiac, we are all gluten free though (well my husband cheats and eats low gluten grains sometimes, he doesn't seem to have too much of a problem). The elder two children are clearly reactive to gluten both behaviorally and physically, my youngest has been raised gluten free and there is no benefit to letting her have it and too much risk in trying so she's not likely to ever eat gluten.
When I say diet hasn't obviously helped with the inattentiveness it's hard to pin down exactly. My eldest (who does not benefit from ADHD medication) still seems like a complete pixie to me most of the time, but many things are much improved and we saw massive improvement in school performance the first year on the diet and it has been maintained. I am not sure what has changed to make her more able to learn when she still seems to spacey, but the performance change was overt. She started the diet in the last weeks of her yr3 school year, she had the same teacher in yr3 and yr5 (so the same teacher saw her after a one year break, during which she was on the diet) and I know her teacher was astonished at the progress DD had made in a number of areas, reading in particular. It's hard to say exactly which factors had the most impact, diet was not the only approach we used with her, but given how she falls apart with a diet mistake I am confident in saying diet was definitely one of the factors.
