Several tests are used to determine the exact cause of allergies, and here are some of the more useful ones:
Method: A liquid containing particles of an allergen is placed on the skin and the skin is pricked or scratched with a needle. You can be tested for up to 30 different allergies at the same time, but this would mean 30 separate pricks. Most people do not have such a large number of allergens tested. Your doctor can select those substances that are most suspicious as possible causes of your allergy symptoms.
When Recommended: When your medical history suggests an allergy but the source of the allergy is not obvious. Also, these tests also can be helpful to help plan treatment if you and your doctor are considering allergy shots (immunotherapy). These tests are recommended to check for allergy after a large local reaction to an insect sting, or symptoms after a sting that suggest allergy (such as throat swelling or wheezing).
Preparation: Stop taking antihistamines two to five days before the test (the length of time depends on the particular drug you've been taking; your doctor can advise you).
Discomfort: Not painful, but can cause itching. The needle doesn't penetrate deep enough to cause bleeding.
Reliability: Frequently helpful for identifying major airborne allergies (pollens, molds, dust mites, animal dander, and cockroach proteins). It can produce positive results for foods even when you do not have a true allergy, however. With the exception of some fruit and vegetable allergies, food skin tests can be helpful when there is no reaction; in that case, you can be reassured that it is about 95 percent likely you are not allergic to the specific food.
Results: If you are allergic, a raised red mark may develop within 20 minutes at the site of the scratch. No reaction means you're probably not allergic. You can have a positive reaction to a substance (especially a food) that you are not allergic to.
Risk: Extremely small risk of an anaphylactic reaction.
Precautions: Because of the risk, a doctor should be present and emergency equipment should be available.
How Done: A diluted extract of an allergen is injected just below the skin. You can be tested for 10 or more allergens at the same time.
When Recommended: This test is generally only used when it would be a problem for your doctor to wrongly identify you as not allergic to something. For example, because insect sting allergies can be life threatening, and since allergy shots are usually recommended after an insect sting allergy is identified, your doctor may use this test to double check the result of a negative skin prick test.
Preparation: Stop taking antihistamines for three to seven days before your test.
Reliability: More sensitive than prick tests, but may indicate allergy to substances that actually don't cause symptoms. Not reliable for food allergy. Best test for drug and bee sting allergies, and occasionally used if an anaphylaxis reaction has been blamed on a possible latex allergy.
Results: If the test is positive, swelling, itching and a raised red mark will develop within 20 minutes.
Risk: The danger of an anaphylactic reaction is slightly higher than with prick tests.
Precautions: A doctor should be present and emergency equipment should be available.
Method: A small patch of absorbent material is coated with an allergen and applied to the back or forearm. The patch is covered with a bandage and left in place for 48 hours.
When Recommended: When allergy is believed responsible for contact dermatitis.
Preparation: Steroid cream or ointment should not be used for seven days in advance. Oral steroids may cause false negative results.
Reliability: Accurate for contact allergies to metals, latex and chemicals.
Discomfort: None, except symptoms that develop from allergen.
Results: Itching or blistering can develop as a reaction to the test allergens, indicating that you're allergic to the substance being tested.
Risk: It is possible for skin irritation to occur, sometimes with lightening or darkening of the skin that persists after healing is done. Rarely, a skin infection may develop during the time that the skin is healing.
Precautions: Usually none.
Food Challenge Tests
Method: You temporarily eliminate from your diet a food suspected to cause symptoms. If symptoms disappear, then you will be given a small, carefully measured “dose” of the food, which can be packaged into a capsule or given in a liquid. Depending upon the type of allergy reaction that you have experienced in the past, it may take you only an hour or two, or several days, to demonstrate whether you have a reaction. If enough time has passed for a reaction to occur and you have remained without symptoms, then your doctor can administer a higher dose of the food product. This test continues until you have had an obvious reaction to the food (so the cause of your allergy is confirmed) or until you have reached a usual portion size of the food with no reaction.
When Recommended: When symptoms aren't typical of allergy but may be due to a food intolerance; when a patient believes he or she is allergic despite a negative skin prick test; when it's necessary to validate the accuracy of a positive skin prick test; when a child who had anaphylaxis from a food has grown old enough that he or she may have “outgrown” the allergy.
Preparation: Eliminate suspected foods from diet for seven to 14 days prior to testing. Stop taking antihistamines for three to seven days prior to testing.
Discomfort: Symptoms can develop.
Reliability: Extremely reliable; the "double-blind" version is considered the "gold standard" of food-allergy testing.
Results: A reaction to the capsule containing the food allergen, but not to a placebo (which is inactive), indicates allergy. If no symptoms occur, no allergy exists.
Risk: A severe asthma attack or anaphylactic shock can occur. The test should only be used for people whose symptoms include anaphylactic shock if other tests cannot clearly identify the allergen, or if an allergist believes that a child may have “outgrown” an allergy.
Precautions: Must be done under medical supervision.
Method: You eliminate one or more foods suspected to be the cause of your symptoms. If you have eliminated several foods at once, you can reintroduce the foods to your diet one by one after your symptoms have gone away to see whether symptoms reappear. (If you have experienced anaphylaxis, it is never recommended for you to reintroduce the suspicious food unless other tests have proven that you are not allergic.)
When Recommended: When only one or two food types are suspected of triggering symptoms.
Preparation: Eliminate suspected foods from diet for seven to 14 days prior to reintroduction.
Discomfort: None except for allergy symptoms upon reintroduction.
Results: If symptoms disappear once the food is eliminated and recur when it's reintroduced, allergy is confirmed.
Risk: Reintroducing the food could lead to an asthma attack or anaphylactic shock, even if you have had milder reactions in the past.
Precautions: Must be done under medical supervision; should not be performed if there is a history of severe allergic reaction.
Method: RAST stands for the radioallergosorbent test, a blood test to measure amounts of IgE antibody specific to a suspected allergen.
When Recommended: When the risk of an anaphylactic reaction makes skin testing too dangerous, or when symptoms are too severe to make it practical for you to stop taking antihistamines. Also, when severe or extensive eczema makes skin testing impractical and when skin-test materials for the suspected allergen are not available.
Discomfort: Minimal when blood is drawn.
Reliability: A positive test makes it very likely you have a true allergy, but doesn’t prove it for sure. A negative test does not eliminate the possibility that you are allergic, because IgE antibodies are not always detectable by this test between allergen exposures.
Results: Usually available within one week.