The diagnosis of food allergy begins with a detailed medical history and physical examination. The doctor will ask lots of questions in order to obtain a list of suspected foods and a thorough description of the reactions. You will also be asked questions to determine whether you have other allergies and whether members of your immediate family have an allergic condition.
Some of the questions the doctor may ask you are:
After this initial evaluation, you may be asked to undergo some of the following tests:
To perform a prick skin test, a tiny amount of solution containing a minuscule amount of a food you may be allergic to is placed on the skin. The skin is then pricked through the drop of extract. If you are allergic to the food allergen in the solution, you will develop itching and a bump surrounded by an area of redness where the skin was pricked. After 15 to 20 minutes, each skin test reaction is measured to document the amount of swelling and redness. A large enough skin reaction is called a positive skin test. This test is useful for identifying which foods may have caused the reaction, but some people can eat the food to which they have tested positive without having an allergic reaction. Your doctor will interpret the results and explain what they mean.
Prick skin testing generally takes 30 to 40 minutes to complete.
Antihistamines and other medicines can affect skin test results. This section has more on how to prepare for allergy skin testing.
Antihistamines and other medications, including those listed below, can affect test results, so check with your doctor or your child's doctor about which medications need to be stopped before skin testing and when to stop them.
Antidepressants can also act as antihistamines. Be sure to let your doctor know if you are on any antidepressants before your skin testing.
A blood test to check for IgE antibodies to a specific food can also help doctors determine if you might be allergic to that food. For some of the foods that commonly cause allergic reactions, such as milk, eggs, and peanuts, a threshold level of specific IgE antibody in the blood has been determined. People with a history of a reaction and an amount of IgE antibody above the threshold level have a high likelihood (95 percent or greater) of reacting when exposed to that food. Tests to determine the blood level of IgE antibodies to foods can be repeated over time to see if the level is decreasing and the food allergy is possibly being outgrown. The results of blood tests measuring IgE antibodies to foods and skin tests to the same foods usually correlate well.
There is some evidence that blood tests are not as sensitive as prick skin tests in identifying Food Allergies. As a result, if the story of the reaction to the food is convincing, but the blood test for IgE to that food is negative, consulting with an allergist for a skin test should be considered. The blood test may be used instead of skin testing if you have a skin condition, such as severe eczema, that makes it difficult to find a site to perform skin testing or if you have trouble stopping medications that might interfere with skin testing. Although the likelihood of an allergic reaction to a prick skin test is extremely rare, in some cases a blood test is chosen to avoid even the remote possibility of causing a reaction.
Your doctor also may consider a food challenge. In some cases, this test is the only way to make a definitive diagnosis of food allergy. The reason to perform a food challenge is to figure out if a suspected food is causing the symptoms by having the patient eat increasing amounts of that food under medical supervision until symptoms occur or the person tolerates a normal portion of the food.
Food challenges should be performed in a medical setting by experienced personnel with the medications and equipment needed for the treatment of anaphylaxis readily available. Before the food challenge, the doctor will review the patient's history. People who have uncontrolled asthma or who are otherwise ill should not participate in a food challenge. Doctors will check vital signs and lung function and perform a physical examination before starting a food challenge and during the test if the patient complains of symptoms.
Food challenges can be done different ways. In one type, called an open food challenge, both the patient and medical staff are aware that the patient is eating the suspected food. For example, a child receiving an open food challenge to egg might be given increasing doses of scrambled egg every 15 to 30 minutes until a whole egg is ingested.
In a single-blind placebo-controlled food challenge, the medical staff is aware of what the patient is being fed, but the patient is not. The suspect food is given as a hidden ingredient in another food.These types of trials are done to prevent the possibility of symptoms caused by patients being worried about eating a certain food. An egg trial done this way, for instance, would involve feeding small amounts of egg concealed in a different food, such as chocolate pudding. Each dose given to the patient could either contain concealed egg or not. However, the final dose of any food challenge is the open ingestion of a normal portion of the suspected food.
In a double-blind placebo-controlled food challenge, neither the patient nor the medical team conducting the test is aware of what the patient is being fed. (For the safety of the patient, at least one physician who is readily available but not directly involved in the test knows what the patient is ingesting.)
A food challenge generally takes between four and eight hours. If a child is having the challenge, a parent should be present for the duration of the challenge. Your doctor may ask you to bring in a specific food for a food challenge. If your child is being tested, you may also be asked to bring a favorite food in which to conceal the food being challenged.
This section has more information on:
Performing food challenges in patients with suspected Food Allergies is often anxiety producing for patients and their families. Indeed, asking someone to eat a food that might make him or her ill seems contrary to that basic premise of medicine (and parenthood), "First, do no harm." However, there are a number of situations in which food challenges make sense.
First, some food challenges are performed to prove that a food is not the cause of symptoms. An example is a patient who has had a reaction to a food and has been labeled allergic despite unconvincing skin test or blood test results. Many people who think they are allergic to a common food try to prevent exposure to that food by markedly restricting their diet. Finding out through a food challenge that they are not, in fact, allergic to the food can be very liberating.
Another common reason for performing food challenges is to see if the food allergy has been outgrown. In the majority of young children, allergies to milk, eggs, soy, or wheat are outgrown over the first several years of life. Thus, performing food challenges may be considered in children who have had accidental exposures without a reaction or whose skin test or blood test reactions have diminished to the point that having outgrown the allergy is deemed to be a reasonable possibility.
Sometimes a food challenge is used to pinpoint which of several foods ingested before a reaction is the cause of the reaction. For example, an adult who has a reaction after eating a large meal in a restaurant might have a positive skin test to more than one food or ingredient included in that meal. Even after appropriate skin testing and testing for IgE antibodies in the blood, it may not be clear which, if any, of the foods was the culprit. Determining which food actually caused the reaction is vital to preventing future reactions and avoiding eliminating safe foods from the diet.
Documenting the degree of sensitivity to an allergen is another reason for performing food challenges. For instance, some patients become concerned that exposure to even minuscule amounts of a food will cause a severe reaction.Carefully performed food challenges can be done to obtain a more accurate measure of a person's level of sensitivity to a tiny amount of the food.
Antihistamines and other medications can affect the results of a food challenge, so you should check with your doctor or your child's doctor about whether and when to stop medications, including any of the following, before a food challenge.
Sometimes antidepressants can also act as an antihistamine. Let your doctor know if you are on any antidepressants before your food challenge.
The purpose of a diet/symptom diary is to record detailed information about food intake and obtain an accurate record of when, how often, and how long any symptoms are experienced. Sometimes, the information is collected over a period of weeks. The information is then reviewed to see if symptoms reliably occur in association with the ingestion of a particular food. When the symptoms are significant and occur infrequently, keeping a diary may be as simple as writing down everything eaten for several hours before each reaction. The information obtained may identify foods that are worthy of further investigation. On the other hand, reviewing the diary may remove a food from suspicion.
Last reviewed on 11/6/09
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