When, Why, and What Does It Mean?
When evaluating for food allergy or other allergic problems, it is not uncommon for an allergist to test for foods that a patient has never knowingly eaten or is eating without obvious problems. This is especially true in children with moderate to sever atopic dermatitis (eczema) or asthma, where acute symptoms may not be readily apparent following eating, or in children with a known food allergy.
For example, infants with milk or egg allergy develop other food allergies in about 35% of cases, and children with peanut allergy will develop allergy to at least one tree nut in about one-third cases.
Negative tests are very reassuring that the food can be ingested safely, except in allergic disorders that are not due to IgE (allergic) antibodies, such as certain types of food allergies that cause symptoms limited to the gut (abdominal pain, vomiting, and diarrhea). The problem arises when a child tests positive to a food that he or she is eating with no apparent problem.
Although no diagnostic test in medicine is 100% accurate, nowhere does the discrepancy between a positive laboratory test and the absence of clinical symptoms seem more apparent than in the various tests used to diagnose food allergy. In large part, this stems from a misinterpretation or “over-interpretation of what the test can actually do.
A positive prick skin test to egg or milk simply indicates that a person has IgE antibodies to milk or egg – that the patient is “sensitized” to milk or egg. The positive prick skin test does not necessarily mean that the patient will experience an allergic reaction to milk or egg.
Overall, less than one-half of individuals with a positive skin test to a food will develop allergic symptoms if they eat that food. The larger the skin test wheal (raised area), the more likely it is that someone will react to the food but no skin test size is 100% definitive.
When determining whether someone is likely to react to a food the allergist must weigh a number of factors, including a detailed history, prick skin test results, and food-specific IgE antibody levels. Even after considering all these factors, the allergist may still not know whether a patient will react to a specific food, and a food challenge may be recommended.
Many parents wonder how their children can have positive skin tests or blood tests to foods that they have never eaten, since you cannot make IgE antibodies against something that your immune system has never seen.
Because many foods are made up of related protein (i.e., botanically related, such as legumes – peanuts, peas, green beans, lentils, etc.), the skin test or blood test may not fully discriminate between various members of food families. Consequently many of the tests will appear to be positive, even though the patient will not react to the food when it is eaten.
The tendency of IgE antibodies to bind to several different related foods is called “cross reactivity.” About 90% of peanut-allergic patients can ingest all other members of the legume family (peas, beans, soy, lentil, chickpeas) without allergic symptoms, even though they may have positive skin tests to many of these foods. About 90% of milk- or egg-allergic patients can eat beef or chicken, respectively, even though their skin tests are frequently positive to both.
Grass pollen-allergic patients often test positive to grains such as wheat, oat, and corn, but they almost always can eat those foods with no problem. Certain pollen proteins are similar to food proteins (such as ragweed pollen and melons and bananas or birch pollen and apple, plum, carrot, kiwi, etc.). Consequently, birch pollen-allergic (“spring hay fever” or allergic rhinitis) or ragweed-allergic (“fall hay fever”) patients may have positive allergy tests to related foods and not experience any symptoms when the food is eaten.
In addition, it is possible that infants become exposed to food proteins from inevitable and unsuspected placed in our environment. Researchers have suggested a number of possibilities: processed foods contaminated with other foods, inhalation of food protein in vapor particles from cooking or in house dust, small amounts of food proteins passed in breast milk, residual food on parents’ or siblings’ hands contacting the skin of babies with eczema, and possibly by contact with food protein in the mother before the child is born.
As noted above, many of these exposures are inevitable, and researchers cannot agree whether some of these exposures – for example, food protein in breast milk or the mother’s system prior to birth – may be protective, rather than harmful.
IgG antibodies are typically made by the immune system to help protect us from infection; however, IgG antibodies also are made against the foods we eat and are normally found in most individuals. Levels of food-specific IgG antibodies may be higher in some individuals who have various gastrointestinal disorders, but this does not necessarily signify an allergy to the specific food.
A variety of other tests are used to “diagnose” food allergy, but at this point, they must be considered “unproven” until well-controlled clinical trials demonstrate their value in identifying specific food allergies.
Although certain allergy tests are very accurate at detecting and quantitating IgE (allergic) antibodies, they must be interpreted by someone who is highly skilled in the diagnosis of food allergy and who understands the clinical limitations of these tests.
Overt he past decade, research has enabled the allergist to diagnose more accurately which patients will actually react to a food and when they have likely “outgrown” their food allergy, but in many cases, the physician-supervised food challenge is still necessary to provide the patients with the correct diagnosis.
Positive skin tests or blood tests to a food that someone is eating regularly with no allergic symptoms, including chronic eczema, hives, or asthma, are never a good reason to eliminate the food from the diet. Only by considering the patient’s history and allergy test results can the physician decide what foods should be removed from a patient’s diet.