Allergy Testing
Tests for Allergic Reactions to Inhalants
To determine if a person is allergic to an inhalant -- that is, an allergen that
can be inhaled, such as pollen, mold, cat dander, or dust mites -- we use a form
of skin testing known as skin endpoint titration. In this procedure, a minute
amount of the suspected allergen is placed under the top layer of the skin on
the upper arm. In contrast to skin-prick testing, skin endpoint
titration shows not only what a person is allergic to but also how allergic he
or she is.
Tests for Contact Allergies (Dermatologist)
We use patch testing to determine the cause of a contact allergy -- that is, an
allergy that results after a person comes into contact with an allergen. Common
causes of contact allergies are makeup and latex. A patch test is used to test
whether a substance that should not be injected into the skin -- for example,
metals such as nickel -- causes an allergy.
In a patch test, the suspected allergen is attached to a nonreactive backing, which holds the allergen against the skin for 48 to 72 hours.
Tests for Food Allergies
An adverse reaction to food can be categorized as either food intolerance or
food allergy. The main difference between these two is that food intolerance
does not involve the immune system; a food allergy does. Food intolerance can
usually be helped by some means, such as by taking a dietary enzyme supplement.
The symptoms of a food allergy may be treatable, but the allergy itself may not
be correctable.
A common example of food intolerance is that caused by lactose deficiency. A person with lactose deficiency experiences discomfort and other symptoms after eating dairy products because his or her body does not produce enough lactase, the enzyme needed to digest dairy foods. A person with lactose intolerance can address the problem by taking a lactase supplement. In contrast, an allergy to dairy products cannot be corrected by taking a supplement.
The eight most common food allergens -- milk, eggs, peanuts, tree nuts, soy, wheat, fish, and shellfish -- cause more than 90 percent of all allergic reactions to foods.
Many tests for food allergy involve testing for immunoglobulin E (IgE). Immunoglobulins are proteins. Most help protect the body from infection. IgE can be different. In about 20 percent of humans, IgE reacts against substances that are harmless, such as pollen grains, animal dander, and some food proteins. It is involved in causing allergic asthma and allergic rhinitis (hay fever) and, in many cases, the symptoms of sneezing, swelling, redness, and itchiness. The people who suffer from allergies are people whose IgE reacts to allergens.
To diagnose food allergies, the Division of Sino-Nasal Disorders and Allergy uses:
Elimination challenge diets. Elimination diets are an essential part of diagnosing food allergies and are used to test reactions to foods that are not necessarily associated with high levels of IgE. In the diet, the patient does not eat a specific food for five days. If the patient is allergic to the withdrawn food, the period of abstinence makes him or her much more sensitive to it. On the sixth day, the patient is "challenged" with the food -- that is, he or she eats a small portion of it. If adverse symptoms occur -- such as itching, gastrointestinal distress, facial pressure, or nasal congestion -- the patient is allergic to the food.
In vitro testing, including radioallergosorbent (RAST) tests. The division uses in vitro testing to detect allergies in patients who cannot undergo skin testing or who have a potentially life-threatening allergy, such as peanut anaphylaxis. In vitro testing is done outside a patient’s body. It involves taking a blood sample from the patient and determining what kinds of IgE and how much of each kind are present.
A radioallergosorbent (RAST) test is a blood test in which a blood sample is placed on a disc that contains specific food proteins. If the person who provided the blood is allergic to that food, the blood develops specific antibodies in response to the food proteins. A high level of antibodies suggests a possible food allergy. A RAST test is very sensitive. If a RAST test shows no elevation in response to a food, then the patient does not have an IgE-mediated allergy to it.
Skin tests. In a skin test, a minute amount of a suspected allergen is placed under the top layer of the skin on the upper arm. If the skin shows a reaction, the patient is allergic to the allergen.