Amagansett - Recent cruising around the allergy blogosphere has turned up an issue I don't encounter much in my practice. How often should allergic children be re-tested for their food allergies? Anxious parents are wondering, is this going to the year our little boy or girl will outgrow them and we can go back to being a normal family?

As far as I'm concerned, the issue of re-testing is an extension of the problem of testing. Tests are good at proving negatives, but they also result in many false positives. A good allergist will never look at a computer print out and declare that a patient must categorically avoid everything that shows some reaction. He will take an extensive clinical history.

As we say in our book: "positive results in response to a food do not mean a person is allergic to the food. Allergy has to be confirmed by a challenge test. The patient may have the [IgE] antibodies, yet be tolerant. A positive result may prompt a GP or pediatrician to recommend that those foods be avoided in addition to the peanuts, say, or milk, where the sensitivity is dangerous enough." We don't want to challenge for everything. It's time-consuming, expensive, and very anxiety producing for patients and their parents. On the other hand, so is managing a very restrictive diet.

What accounts for the large numbers of false positives? In great part it's because proteins that can be potent allergens in some patients can also be found in other items. Nature doesn't like to reinvent the wheel as new species evolve from common ancestors. Take shellfish, for example. Orthodox Jews are prohibited by religious dietary laws from eating shellfish, yet often test positive to crustacean allergens, probably due to exposure to non-crustacean species, such as house dust mites (HDM), cockroaches or both. This is called cross-reactivity. The same thing can happen with certain fruits, nuts, and legumes when the real problem is the pollen from a tree in the neighborhood.

Once I am convinced a food allergy is real, as time goes on, I don't routinely check to see if children are "still" positive. I take my cues from the patient. What usually transpires is that Mom will bring the child to the office and report that he ate (whatever) and nothing happened. With something like ice cream for a child who is egg or dairy allergic, the child will usually indulge himself pretty thoroughly before getting up the courage to tell Mom and Dad. That doesn't usually happen with things like kiwi.

I will then do a modified food challenge - the gold standard double blind placebo food challenge is pretty arduous. An experienced allergist can offer a middle way by applying a sample of the food to the skin, taking before and after pictures. If the first application doesn't react, we repeat the process on the lips without letting the patient ingest it. The final stage is full ingestion, with the doctor at the ready with epinephrine. By the time we get to this stage, we are pretty sure there will be no harm. If the results are negative, both patient and parents won't automatically incorporate the new freedom into the diet. They remain wary, having been so disciplined about eating the food, they might avoid it out of residual fear or habit, not out of necessity.

Is there an answer to the testing dilemma? I must answer this with a qualified yes. While nothing will ever substitute for the clinical judgment of a good allergist for both diagnosis and treatment, there is a new technology called component testing that could supply the missing link. They test for the individual components of known allergenic proteins. Unlike old blood tests, or even skin tests, they show whether a patient is only reactive, mildly allergic, or dangerously allergic. Last month, I had a child come to my office who had always avoided eggs because his pediatrician said he was allergic to them. The component test showed he didn't have a problem. Now breakfast is his favorite meal.