My friend is allergic to peels. He can eat apples, almonds, zucchinis, eggplants, potatoes and many other nuts, fruits, vegetables normally. But only when they are peeled. Another friend of mine was allergic to the kitchen garden plants but only when cooked. You know, like lemongrass, parsley, mint etc. - the ones you grow in the kitchen. It took them little over 8 months to figure this out. Maybe this is something you could try if you haven't already.
But to answer your question, they are fairly accurate. They find out patterns of allergens and round-up on foods that you need to avoid. My colleague found out that he is seasonally allergic to a particular brand of milk as it turns out the milk comes from cows that eat a particular type of grass and hay, typical of the region where the milk is produced. So for the months of March-May/June, he shifts his brand of milk to something that's produced from cows with a more diverse intake. He hasn't had the problem since. He was able to get this done with the help of an allergy test.
