This article first appeared in the AllergyMoms Newsletter May 17, 2007.
To diagnose a food allergy, your doctor is likely to ask a bunch of questions (take your history), examine you, and run one or more allergy tests. One of the pressing questions for both patients and doctors is “Which test is best?” Can your doctor diagnose food allergy by simply talking to you? Are RASTs or skin tests more accurate? When is a food challenge appropriate? These are all excellent questions that often puzzle both patients and doctors. Although every individual case is different, and I cannot provide specific medical advice, I can provide some insight that may help you and your doctor decide which tests are best in your situation.
The History
In some instances the patient’s medical history is virtually all that is needed to diagnose a food allergy- you eat peanut butter for the first time, and your face swells up to the size of a basketball. In other instances, however, when the allergic reactions are more chronic or delayed, as in the case of atopic (allergy-related) eczema or gastrointestinal food allergies, the history may be almost useless. We would take a history to know what is in your diet, but we would not necessarily be able to see any cause and effect between your symptoms and what you have most recently eaten.
Skin Tests
Skin testing is used only in the diagnosis of IgE- mediated food allergies-allergies in which your immune system produces IgE antibodies to a specific food. IgE-mediated food allergies include most cases of hives, anaphylaxis, and eczema, while many gastrointestinal food allergies do not involve IgE antibodies.
Note: IgE (Immunoglobulin E) is an anti-body that the immune system produces to fight enemy invaders. When you have an IgE-mediated food allergy, your immune system has higher than normal levels of IgE targeting the food that triggers your symptoms. Here are four important points to keep in mind about the pros and cons of skin tests:
- If you have symptoms of IgE mediated food allergy (based on your history and physical examination), a skin test can be very helpful in identifying the problem
food(s). - False-positive tests are very common, so just having a positive skin test does not automatically mean that you are allergic to that food. All tests need to be carefully interpreted by an allergist with expertise in food allergy.
- Skin tests are of no use in identifying a non-IgE mediated food allergy. For diagnosing a non-IgE mediated food allergy, a food diary or diagnostic elimination diet would probably be more useful.
- Skin test results can be affected by medication, so you have to stop your allergy medications for some time before the scheduled tests.
If your doctor suspects only food allergies, she should not have to test for 20-40 different foods or substances. Testing for five to ten of the most common food allergens, including milk, egg, soy, wheat, and peanut, is usually sufficient.
RASTs
Short for short for radioallergosorbent tests, RASTs look for the presence of food-specific IgE in the blood. If you are allergic to milk, for example, you have milk IgE in your blood, indicating that your immune system has identified milk as an enemy invader. Generally speaking, the higher the level of food-specific IgE in the blood, the more likely you are to have a reaction to that food and the more severe the reaction is likely to be.
The following items explain the pros and cons of RASTs and point out some important points to keep in mind:
- Unlike skin test results, RAST results are not affected by the presence of medications, which is a big plus if you cannot stop your medications in order to be tested for food allergies. They may also be preferable in patients with severe eczema in when there may not be enough normal skin to do skin testing.
- As with skin tests, false positive tests are very common. However, when the RAST result is above a certain level, they are highly reliable-they can predict with about 95-percent accuracy whether a child is allergic to egg, milk, peanut, tree nuts, or fish.
- RASTs are very useful in determining when conditions are safe enough to warrant a food challenge in order to determine whether you have outgrown your food allergy.
- RAST results can be tricky to interpret, so be sure you are working with an allergist who has the knowledge and experience to decipher the test results and determine the appropriate course of treatment.
Food Diary
When you have a non-IgE mediated food allergy or reactions that your doctor cannot trace back to a specific food using either a skin test or RAST, then your doctor may ask you to keep a food diary listing the foods you eat and drink and recording any reactions to those foods. A food diary can be useful in identifying overlooked foods, hidden ingredients, and patterns of reactions.
Diagnostic Elimination Diet
A diagnostic elimination diet simply removes selected foods or food groups from the diet for a period of time to determine whether symptoms will disappear. If you stop eating anything with egg in it, for example, and symptoms go away, it is likely that you have egg allergy.
While this sounds easy enough and certainly less painful than getting poked with one or more needles, the diagnostic elimination diet does have a couple drawbacks:
- Removing an essential food from the diet of a growing child could raise some nutritional issues.
- If the patient is not skilled in reading labels or vigilant enough, she may be eating enough of the suspected food to make the results completely useless.
Food Challenge
The only 100-percent reliable test for a food allergy is a food challenge-eating the suspect food under your allergist’s supervision. If you eat a suspect food and you react, that sends a clear, unambiguous message that you are, in fact, allergic to that food.
The obvious drawback of a food challenge is that it can be highly risky, and is certainly not something a patient should even consider trying on her own. Only a qualified allergist who’s familiar with food-allergy reactions and is equipped with the necessary emergency medications should perform a food challenge.
In my practice, I often use RAST results along with a food challenge to determine whether a patient has outgrown an allergy. First, I look at the RAST results. If the food-specific IgE has dropped to a concentration that shows at least a 50/50 chance that the person has outgrown a food allergy, I consider the potential benefit high enough to warrant the risk of a food challenge.
What are the food-specific IgE levels we look for? For IgE-mediated milk, egg, peanut, or tree nut allergies, we look for levels of food-specific IgE to drop below 2kUA/L before we deem the risk level acceptable for moving forward with a food challenge. For wheat, soy, and other foods, no “safer” levels have been clearly established.
Remember: Although I offer some general testing guidelines here, this article is no substitute for professional medical diagnosis and treatment. Consult with your medical doctor to obtain the tests and treatments that are most appropriate for you or your loved one.