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Food Allergies

How Do I Find Out If I Have Food Allergies?

Last updated on:
22/03/2012

Contributing Author: Guy Slowik FRCS

If you think you have food allergies, you should see an allergist, a physician who specializes in diagnosing and treating allergies. If you try to diagnose the problem yourself, you run the risk of removing too many foods from your diet and losing important nutrients.

Before making a diagnosis, an allergist will take your medical history and ask whether you have a family history of allergies. The doctor will ask detailed questions aimed at uncovering a possible relationship between the symptoms and your diet, such as:

  • What kinds of symptoms does the suspected food or foods produce?
  • How much time elapses between eating the food and the onset of symptoms?
  • How much of the food do you have to eat to trigger a reaction?
  • How many times has eating the food caused a reaction?
  • When was the last time you had a reaction to the food?

An allergist will also use diagnostic tools, including:

A Food diary

Elimination diet

Skin tests

Blood tests

Food challenge

Bowel biopsy

A Food Diary

A food diary is a daily list of all the foods you eat, when you eat them, any symptoms that occur and how long between eating and symptoms

A person with a suspected allergy may be asked to keep a food diary for a few weeks, recording in detail the foods eaten and the appearance and severity of symptoms experienced.

While the food diary cannot prove that a particular food causes certain symptoms, it can indicate possible links between foods and symptoms that might otherwise go unnoticed.

It may be more helpful to maintain an “event" diary. When symptoms occur record all the food eaten at the previous meal. This may identify the problem much more quickly and with less effort.

Nice To Know:

If you suspect you have a food allergy, start a food diary before your first appointment with your allergist. It is particularly helpful to make a list of foods eaten in the few hours before a reaction.

Elimination Diet

An elimination diet is one from which foods suspected of causing an allergic reaction are removed for as long as several weeks. The physician usually provides a list of foods that are permitted or forbidden during this time.

  • You will be asked to eliminate the suspected foods for about two weeks. (You must be certain that a diet lasting longer than two weeks is nutritionally adequate.)
  • You'll note whether your symptoms disappear during the diet.
  • In some cases, the doctor will ask you to reintroduce the foods back into your diet one at a time, and you'll note whether symptoms return after you start eating the foods again. This reintroduction process is similar to a procedure known as a food challenge.

Need To Know:

Consult your doctor before beginning an extensive elimination diet. A doctor's supervision will insure that you do not remove too many foods from your diet.

Long-term elimination diets are also a treatment of food allergy.

Skin Tests

Skin testing can help pinpoint which foods trigger allergic reactions in people who have a true food allergy. It can also help rule out a food allergy when the problem is actually a food intolerance.

Skin tests are useful in detecting food allergies, but they are not foolproof. Allergy skin tests detect only the presence of antibodies. They cannot make an accurate determination that an allergy exists.

The least expensive allergy skin test is the skin prick or puncture test. The physician places a droplet with an extract of the suspect food on the skin, and then uses a needle (or special device) to puncture the top layer of skin through the droplet. (If the physician scratches the skin through the liquid allergen, it's called a scratch test, a test that is rarely used now.) In recent years several new skin testing devices have been introduced into allergy practice.

If a raised bump surrounded by an area of redness (known as a wheal or hive) forms within 15 to 20 minutes, the skin test is positive. In other words, the person has an antibody for that allergen.

  • A positive response does not necessarily mean that the person has a food allergy. Many people, especially those with a family history of allergy, can have a positive test result and experience no allergic symptoms when they eat the food. You do not have an allergy unless you have symptoms as well as an immune response to the allergen.
  • When there is no reaction to the allergen, the skin test is negative. A negative result almost always means the person is not allergic to the most common food allergens. Sometimes, however, a person with an allergy will still test negative. This is called a false-negative result. A false negative may happen, for instance, in young infants with poor skin reactivity, or if the skin testing material is not reliable.

Nice To Know:

To test reactions to fruits, vegetables, and some other foods, physicians use a "fresh food" skin test. There are several techniques for accomplishing this procedure, but it always involves fresh food.

In some cases, a physician will inject the allergen under the patient's skin. Doctors do not use this "intradermal" (within the skin) test as often as the skin prick test because it has a greater chance of causing a severe, systemic (whole-body) reaction if someone is very allergic. It is also less accurate than the skin prick test.

Blood Tests

A blood test called the radioallergosorbent test (RAST) is also useful in the diagnosis of food allergies. It measures levels of IgE antibodies specific to certain allergens in a blood sample.

The IgE RAST does not have the side effects associated with the skin prick test (itchy hives). However, the RAST is more expensive and less immediate than the skin prick test. Individuals must wait to get test results back from a lab. It is also less sensitive than skin testing most of the time, and sometimes the quality control of the test is a problem. A new version of the RAST (CAP RAST) may give more accurate diagnosis for a few foods. (Recently it has become possible for individuals to have blood tests for allergy ordered and performed with minimal or no physician involvement. However, making changes to the diet without skilled interpretation and advice may be harmful or even dangerous. It may also make life needless unpleasant by removing foods from the diet that are not causing harm.)

Food Challenge

The only test that can definitively diagnose food allergy is a procedure known as a food challenge. In this procedure, the person is given either a food suspected of causing allergy or a placebo, a food that will not cause an allergic reaction. In some cases, the suspected food is hidden in another food or in capsules so that the patient cannot recognize it.

It is often a double-blind procedure, in which neither the person nor the physician can distinguish the food from the placebo, thus ruling out the possibility that the person's or the physician's expectations are influencing the results.

A double-blind food challenge rules out the possibility of a psychological reaction (when symptoms occur because the individual thinks he or she has a food allergy).

Nice To Know:

Studies show that less than half of all people who suspect they have a food allergy, test positive for food allergy in a double-blind food challenge.

Need To Know:

People who have had severe food reactions are allowed to participate in food challenge only under very special conditions because the procedure itself could be life-threatening.

Bowel Biopsy

In rare instances, a food allergy or food intolerance may cause intestinal damage.

biopsy of the small bowel (intestine) may be performed on people who have frequent diarrhea and cannot absorb certain nutrients. The biopsy is especially useful for diagnosing food-allergy due togastroenteropathies (inflammation of the lining of the stomach and intestinal tract) and celiac disease, which damages the intestinal lining.

Biopsy of the large intestine is helpful in children with food-induced enterocolitis (inflammation of both the colon and the small intestine) and colitisinflammation of the colon. Colitis is a condition in which drinking milk can cause diarrhea and blood in the stools.

 
 

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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.