The challenges my kids have done are similar to what hezz described. They start by checking your lungs, nose, ears, skin, etc. Start with a small does of the food, wait, check you over, have a larger dose and so on. Once done, we had to wait and hour or 2 for the final word that it was a pass. When DS reacted to the baked egg muffin, they had to wait an additional amount of time before they were allowed to leave.
Definitely just try to be calm and if anything feels off, let them know ASAP. If you have questions, call them ahead of time, I am sure they will walk you through everything. I hope the waiting isn't too stressful and it all goes well!