Just a heads up. Not every dermatologist is familiar with dermatitis herpetiformis nor knows how to correctly biopsy it. One common mistake made is to take the sample from the pimple instead of next to it. I agree with Scott. Your reaction to gluten is very strong and you putting your health at risk by trying to go forward with a proper gluten challenge. If it looks like a duck, walks like a duck, and quacks like a duck then it's a duck!  I don't know where you live but in some countries (like the UK) there can be government subsidies to offset the cost of buying gluten free food and other benefits for celiacs like follow up care if you have an official diagnosis. At any rate, talk to your doctor about getting the gene test as knitty kitty suggested and getting diagnosed on what info you do have so far. And if the dermatology biopsy is positive for dermatitis herpetiformis that is gold standard proof of celiac disease. But IMO, destroying your health by going forward with the gluten challenge is too big a price to pay in order to get an official diagnosis.

If your reaction to gluten is this strong and obvious, do you really need a formal diagnosis? Why not just go gluten-free?

Thank you, I will ask the dermatologist for a biopsy.

@BadHobit, You don't need to eat gluten to have a DNA test for Celiac Disease genes.  You need to let your doctors know how severe your reaction to gluten is.  It's endangering your health.   I could not finish a gluten challenge after having been gluten free for a while beforehand.  My doctors diagnosed me by positive genes for celiac disease and improvement on the gluten free diet.  You've got positive blood tests as well.  A baseline endoscopy can still be done, just without the gluten challenge.   The dermatologist needs to take biopsies from skin adjacent to the skin lesions or blisters, not right on top of them.  Hopefully, your dermatologist is experienced with Dermatitis Herpetiformis.   Hope you feel better.  Let us know how things turn out.

Thank you very much for your support and information. I came close to eating a slice of bread yesterday but had to take a stomach pill because it caused pain. I went to the toilet 5 times, unbelievable. And I felt drunk, stumbling left and right and getting quite clumsy. The foul-smelling stools started right away. And I was very restless in my sleep, I woke up several times with nightmares. I don't even count the itching. It's too bad it happened so fast. I have itching and blisters, so I'll see a dermatologist tomorrow. I will let you know if it is dermatitis herpetiformis. I just want to ask. I can't even eat 4 slices of gluten-free bread a day. I can get by on one or two slices of bread or three cookies for six weeks, right?