By Dr. Paul Ehrlich
One of the recurring themes in diagnosing and managing food allergies is confusion over the meaning of different tests. After studying the test results, parents, patients, and doctors still can’t answer basic questions: is my child allergic? Is she dangerously allergic? Is he likely to outgrow his allergies? Do we have to turn our existence upside down?
This confusion is not confined to allergies. We have all read about the proliferation of indiscriminate testing and cover-your-ass medicine, which have played such a noisy part in the health-care reform debate. As an older physician who trained before limits on resident hours and the explosion of modern technology, I worry about general practitioners and specialists who rely too much on numbers and not enough on observation and clinical experience.
As we have pointed out in our book and on this website, placing too great faith in a few numbers can result in narrower diets than are warranted, failure to thrive, and chronic anxiety. If allergists are to be faulted in this, it’s for neglecting to explain the strengths and weaknesses of the different tests, and also for not resorting to more food challenges, which, as we have written previously on this website, are a badly underutilized resource.
So in the interest of greater clarity about the repertoire of tests, my cousin and I have looked to baseball, which in our family is a metaphor for everything. The tests are:
1) Total IgE, the allergic antibodies that float in the blood stream and can be measured using a blood sample;
2) Allergen-specific IgE, which also circulates in the blood and indicates that you are manufacturing antibodies to certain whole allergens, such as peanut, but the numbers don’t mean you will have a reaction—they have to be attached to mast cells and basophils;
3) Skin-prick tests, which can show reactions to mast cells in the skin, but don’t necessarily indicate severity of reactions upon ingestion;
4) Component tests, which indicate whether you are allergic to individual epitopes of food proteins, some of which are associated with severe reactions and some less;
5) And finally, the food challenge, in which you eat some of the food to see if you react.
To us, total IgE is like total at bats during a season. You can learn how many times the batter has come to the plate, but you don’t know whether he has gotten any hits.
Whole-allergen-specific IgE tells you how many hits, but it doesn’t differentiate between singles and extra base hits.
Skin tests show you how many of each kind of hit, but not whether those hits are contributing to team success by driving in any runs.
Component tests show you runs scored and runs batted in, but not whether it is helping the team win, which is essentially the only statistic that counts.
Test results, like batting statistics, are ambiguous. Teams can win without scoring lots of runs, and they can lose when they hit home runs by the bunch. You can be tolerant of a food with high specific IgE or large skin reactions, and you can have anaphylaxis when many of the statistics are favorable. As with baseball, in which walks, stolen bases, and pitching make a difference, allergy has lots of intangibles.
That is why a food challenge is the only test that counts in the standings.
An experienced allergist doesn’t consider these tests in isolation from one another. A combination of results may indicate whether we will take a chance on a food challenge. A combination of skin prick tests and allergen-specific IgE can be used to predict food allergy in some patients. Whole peanut allergen IgE RAST followed by testing for the component Arah2 can help rule a food challenge in or out. An algorithm by Irish and British researchers called the Cork-Southampton Food Challenge Outcome Calculator, which combines skin prick test responses, serum specific IgE levels, total IgE levels minus serum-specific IgE levels, symptoms, sex, and age seems to predict the outcome of oral food challenge with a high degree of accuracy, both positive and negative, although how well the average allergist would do with it is anyone’s guess. {Note: the calculator will eventually be available online for professionals to use.}
All food allergy testing is problematic, up to and including the “gold standard” double-blind placebo-controlled food challenge (DBPCFC). If allergists in private practice are reluctant to do oral food challenges because they don’t pay well, the time they take, and risk of adverse events, imagine how much worse it is for DBPCFCs, which are generally only offered in hospitals (which have a knack for squeezing more money out of insurance companies than private practitioners do, I might add). I have only done one in my practice, but the biggest problem was not the time and the money, it was finding something that could mask the smell and taste of peanut butter so the patient wouldn’t know the difference.
With all the shortcomings of individual tests and combinations thereof, I don’t think we’ll settle this question soon. The pages of allergy journals are always full of the tension between the conventional wisdom and the unknown, between what looks good on paper and reality. Sort of like the sports pages.
(If you don’t understand baseball either, we’re stuck.)