The diagnosis of food allergy rests on a careful evaluation of the patient together with laboratory tests.
Taking a good history may reveal reproducible symptoms involved in eating certain food types. This is usually more obvious in the immediate reactions then in the delayed type reactions.
In immediate type allergies skin prick tests and/or blood tests (which detect IgE antibodies to various foods in the blood) are useful. Remember that a positive skin or blood test does not necessarily mean the person is allergic. If the diagnosis remains uncertain, or to test whether someone has outgrown their food allergy, an oral food challenge may be necessary. In an oral food challenge, initially tiny then increasing amounts of the food in question are given under close supervision to check for reactions.
Laboratory tests are less useful for the delayed type allergies. For such allergies, the final mainstay of diagnosis remains the demonstration of relief of symptoms on removal of a given food item for a few weeks, and recurrence of symptoms on its re-introduction (elimination-challenge testing). A Dietician trained in allergies will need to be involved in this process. In a few cases, a biopsy specimen of the gut may be required.