When I was nine years old, I had a allergy test that involved a nurse putting about 100 scratches on my back and applying a potential allergen to each scratch. If I remember right, I was allergic to about 90 things. I took weekly allergy shots for years after that, completely desensitizing me to needles. But the allergies never really left.
It's the wrong time of year for allergies, but I've had them over the past few weeks. I used to just take a Sudafed and ibuprofin and suffer until the pain went away. Lately, though, I've noticed that allergy attacks happen when I eat something different: non-dairy creamer, a particular brand of sausage (which probably has something that's not on the label), and a cookie (which contained wheat).
A test a few years ago showed that I don't have celiac, a condition where gluten (a protein in wheat) damages the intestines. But you don't have to have a permission slip from your doctor to eliminate things from your diet.
A few months ago, I eliminated everything but meat, eggs, nuts, greens, spices, baking cocoa and some condiments (like vinegar and mustard) from my diet. I felt great! By slowly adding back items that can be part of a low-carb plan, I've seen what I can and cannot tolerate. Basically, I ate a paleo diet (with a few exceptions like vinegar and cocoa), the diet humans and their ancestors were on for two million years. Then I added back technology dependent foods like the cookie (our hunter-gatherer ancestors didn't grow or eat grains). Then I paid attention to how I felt. No expensive, uncomfortable allergy test required.