By Dr. Paul Ehrlich
Someone asked me recently if I thought repeat component testing would be useful for predicting when and if a patient would ever outgrow a food allergy. I must admit I was taken aback. Much as I like and admire these tests—I was an early adopter of the tests made by ThermoFisherScientific (formerly Phadia) and advise the company about my ongoing experience—I think it’s too early to say. As tools for diagnosing food allergies to begin with and gauging the severity of the allergy, the tests are still unfinished business, let alone using them as a crystal ball.
No single test is considered diagnostic of a food allergy except an oral food challenge (OFC), which obviously you don’t want to do indiscriminately because of the risk it poses to the patient. As I wrote a couple of years ago, large numbers of allergists never even did them in their training, and many of the rest are reluctant to do them because of the time, expense, and possibility of an adverse event involved, so they are a scarce service regardless. I do them regularly to confirm tolerance and would do more if parents would agree.
Of the stepping-stone tests, RAST, which measures allergen-specific IgE in the blood, doesn’t diagnose allergy, only sensitization. Skin prick tests show reactivity, but not the extent of the allergy–i.e. the skin may react and wheal sizes are significant, but it may not translate into a measure of other reactions, such as hives, cardiac, or respiratory symptoms. Component testing shows which epitopes within the proteins you are allergic to, some of which, in the case of peanuts, are more dangerous than others, generally speaking. Of the three most widely associated with anaphylaxis, Arah1, 2, and 3, #2 is considered the bellwether. A high Arah2 number is probably a good predictor that the peanut allergy will not go away, and that won’t change with repeated testing.
As we know, the different allergens have general guidelines for permanence. Most milk allergy – an estimated 80%–disappears by the age of five, but if it lasts longer, it’s likely permanent. Peanut? Smaller numbers are outgrown—around 20%. Certain numbers are useful and in the event that a diagnosis is made, those baseline numbers can be predictors of longevity. If they are high, chances are the allergy will last. If they are lower, testing IgE at yearly intervals might show a downward trend. A food challenge might be in order after a certain age and with successful avoidance although it is always an educated guess.
But just as no test (except for a food challenge) is considered diagnostic in the beginning, they aren’t always good predictors along the way. Absolute numbers do not always coincide with the severity of reactions.
Might component testing be predictive of permanence, and if so, will regular component testing be useful? Maybe, but at what intervals? There are no guidelines. I started using these tests probably 10 years ago and I continue to use them, but only if I think they will help solve the puzzle. I have never found them as useful for anything as my own clinical judgment. Certainly there is no hint that the tests could turn any primary care physician into an allergist, as some of my colleagues feared. Most allergists have only started using component tests within the last five years, if they use them at all. I don’t think that’s enough time to arrive at a consensus about how often to retest. I think the best thing about component testing is the ability to help kids with food allergies incorporate a wider variety of foods in their diets. Allergists (and some dermatologists) still see cases of failure to thrive because a pediatrician has decreed overly broad avoidance programs based on lousy use of diagnostic testing. To the extent that we can broaden their diets and give them greater latitude on living kid-style lives, we are doing what we are supposed to do.
Regardless, as long as avoidance remains the primary strategy for managing food allergies, frequent testing is just an exercise in wishful thinking. B-memory cells have long memories. If the numbers are high, the programming is going to last a long time. It’s not going to change right away.
However, that may change as more active treatments come into use, whether a form of immunotherapy, Chinese herbal medicine, or something else. Then easily measured bio-markers such as blood IgE may be useful for measuring progress, as well as components, and more esoteric markers like basophil activation may become more commonplace.
I have nothing against hope, but it’s important to separate hope from wishful thinking, especially if wishful thinking is expensive.