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The Informed Parent

To Test Or Not To Test — That Is Not The Question

by John H. Samson, M.D., F.A.A.P.
Published on Mar. 07, 2011

Allergy patients in a pediatric practice can prove to be very gratifying---or frustrating. Establishing an effective plan that the family will follow is the challenge.

Just as perplexing is the interpretation and use of testing modalities. The time-honored intradermal skin tests are unpleasant for the patient. The blood (RAST) tests involve only one venipuncture for all the tests. But some physicians feel too many false positives may be given.

In reality the useful accuracy of both the skin and RAST tests are essentially the same. It is generally thought that they are about 85% accurate in depicting the offending allergens, whether airborne or in foods. Therefore, the incorrect question is to test or not to test, but rather how to use the results.

The question fits both types of allergens. For example, the tests show a positive reaction to grass, yet the patient has no symptoms during grass season. The result is clearly a false positive. On the other hand, a patient that has severe symptoms during the grass season is allergic to that pollen even though the test was negative.

This issue is more important with foods. There have been several articles in the press lately about over diagnosing food allergies by relying on tests only. They are correct. If the physician takes the test results only and restricts the child’s diet there is a chance that more foods will be removed from the diet than is necessary.

Here is an example case that clearly depicts what happens. Jimmy has asthma on a daily basis. Food sensitivity is suspected. RAST tests are drawn to see what he is allergic to. The statement should be, to see what he MIGHT be allergic to.

Therein lies the problem. We should not take the results as concrete statements of documented allergy. We should use them as guidelines to clinically establish allergens. Let’s say, in Jimmy’s case, he has positive tests to milk, eggs, wheat and soy. To remove all of these food from his diet makes feeding him a real chore. If he is truly allergic to all four we must construct a nutritious diet free of them.

If some of these are false positives he will have an unnecessarily restricted diet. How do you avoid this pitfall? We use the results as indicators of POSSIBLE food allergens. With a list of milk, eggs, wheat and soy one would first remove milk and observe his symptoms over three weeks. If there is no improvement a second food will be withdrawn from his diet, i.e. soy.

It is important to continue eliminating milk from his diet because he may be allergic to both. Therefore, the reintroduction of milk would provoke symptoms. He should then go three weeks without milk or soy. If this proves unsuccessful the same process is established with wheat and egg products.

For discussion sake let’s say Jimmy becomes symptom free with the final, fourth food withdrawal. This means it could be all four or only the last one. At this point we introduce the foods carefully back into his diet over three week periods and watch for the emergence of symptoms. This may seem arduous but you must correlate skin tests or RAST tests with seasonal histories for airborne allergens or with food withdrawal and reintroduction of the potentially sensitive foods.

If you have questions regarding the interpretation of allergy tests discuss them with your pediatrician. Be sure that you treat the patient, not the laboratory results. 

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