I would say “yes.” Whereas not all ADHD cases can be traced to diet, many can. In a 1992 study of 185 hyperactive kids, a low allergy diet paired with calcium, magnesium, zinc and vitamin supplements improved the behavior of 116 kids. Additional, Dr. Benjamin Feingold, founder of the infamous Feingold Diet, claims that 50 percent of his patients with ADHD improved on an elimination diet free of artificial colorings, artificial flavors, salicylates (a naturally occurring preservative), artificial sweeteners and preservatives.
Maybe not "diagnosis" per se, but I think allergies definitely should be considered as potential causes of ADHD. There are so many unknowns, it's hard to figure out exactly what may or may not be causing or exacerbating the condition. It doesn't hurt to find out!
While allergy testing may uncover allergies -- or at least the classical IgE allergies -- it is important to remember that those additives eliminated on the Feingold Diet generally do not involve an allergic condition. Dr. Feingold, as Chief of Allergy at Kaiser Permanente, would certainly have been able to recognize an allergy, and he said that this is a sensitivity, not an allergic condition; you may call it a pharmacological reaction to the additives. I'll say that another way .... considering that the additives are low molecular weight chemicals and similar to drugs in structure, the physical and behavioral reactions to them might be considered similar to the side effects one can get from medications. While Dr. Feingold asked the FDA to study these additives in the same way that drugs are studied before allowing them into the food supply, this never happened. In fact, the artificial flavorings are mostly not even been tested for safety, and none have been tested for neurotoxicity. The only way to know if the Feingold diet will work for you or your child is to try it. Dr. Feingold's early work showed that about 50% of the children responded to the diet; after he also eliminated the petrochemical preservatives, more than 70% responded, and the Feingold Association sees the same today.