I went to a seminar yesterday. It was mentioned that food allergy prick testing is most effective for nut allergies, but other foods do not always show up if you are allergic to them. This means you might be allergic to foods you do not know you are allergic to.
As far as patch testing at dermatologist, I think they could miss a severe allergy if they do not note welts you get on your skin like my dermatologist who only noted any small red spots directly over allergen patch. So, it is best to look at your back throughout the week whenever it itches to see if there are any welts.
How long have you had EC? It is easy for doctor to say it will go away if you did not have it for long time.