Food Allergy Blood Test (IgE)
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Food allergies account for about 5% to 10% of all food reactions. Symptoms associated with food allergies range from mild symptoms such as sinus congestion, watery eyes, hives and itchiness to more extreme symptoms such as wheezing, diarrhea, difficulty breathing or swallowing and in the most extreme cases, food allergies can trigger an anaphylaxis reaction that can be fatal.
Testing for Food Allergies
There are various ways of assessing for food allergies, each one with their own pros and cons. The various methods include:
IgE antibodies are found primarily in mucous secretions. In serum it is present in very low concentrations but may be elevated in atopic diseases such as allergic asthma, hay fever and atopic dermatitis.
An IgE reaction occurs immediately after exposure to the allergen; food or inhalant. This type of reaction is referred to as a Type I immediate hypersensitivity reaction. IgE antibodies that are specific for the allergen, bind onto immune cells called mast cells and basophils. The allergen latches onto the mast cell-bound IgE antibodies in a cross-linking manner. This initiates the release of histamine and other inflammatory mediators from these cells.
The allergic IgE response is comprised of not only an immediate event but in many cases, a late event as well. The early phase reaction usually occurs within 15 minutes of exposure to the allergen. The late phase reaction may then occur 4-6 hours later and persist for days with increased edema and inflammation.
IgE Blood Test
Because of the immediate reaction of the body to food allergies, it is often possible to determine the specific foods after a detailed history is taken or by tracking a person's diet and their symptoms. When further testing is required a food challenge or the skin prick test is considered the gold-standard and most accurate way to determine a food allergy.
The IgE blood tests, sometimes referred to as RAST test, are often used in combination with skin tests, or in situation when other tests are considered risky (e.g., when a person has experienced a severe allergic reaction to a food or other allergen). The IgE blood test is considered an accurate and useful test for detecting the level of specific IgE concentrations in blood., 
- The advantage to the IgE blood test is that it is less risky than that skin prick test, food challenge or food elimination diet.
- Can be used safely when there is a history or concern of adverse reactions such as anaphylaxic, hives or severe eczema.
- Involves a simple blood draw that only takes a few minutes.
- Blood tests for food allergies are less sensitive than a food challenge or skin prick test.
- Measures the level of IgE concentrations in blood. If a person has been avoiding specific foods for awhile than the blood test may show a decreased response.
- Although the range of foods that can be tested is large, you are not able to test for all known antigens.
- Results are generally confirmed with a food challenge.
- Tests can be expensive (a couple of hundred dollars or more).
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