Allergy tests are not perfect tests. A batch of allergy tests has the potential to produce several false-positive results (results that suggests you have an allergy, even though you really don’t) and can have false-negative results (results that suggests you don’t have an allergy, even though you really do). Even so, allergy tests that have positive results can offer some guidance about what you may want to avoid in an effort to prevent allergy symptoms. They also serve as the basis for allergy shots in people who require this form of treatment.
All tests except the RAST require that you stop taking antihistamines and certain other medications for at least three to seven days before testing. Do not discontinue any medication unless your doctor advises you to do so. If your symptoms are too severe to permit a break in your medicines, or if your doctor feels that the risk of a serious allergic reaction to skin testing is too high, the RAST can be used while you are taking medications. The results of the RAST test may be less accurate than results of allergy skin testing.
Skin prick or scratch test.
In the skin prick or scratch test, one drop each of several different diluted allergens are separately placed on the skin, often in a grid pattern on the back. Then the skin is scratched once at the location of each droplet, to increase exposure to each of the allergens. After about 20 to 30 minutes, if redness, hives or swelling surrounds any scratch, you may be having an allergic reaction to the allergen being tested. If manyallergens need to be tested, your doctor may do your testing in batches on two separate days. You will be observed for at least 30 minutes after the test is completed to make sure you don’t develop any breathing problems or anaphylaxis, which requires immediate treatment.
Skin patch test.
Allergens are applied to the skin in a skin patch test. Unlike a skin prick test, a skin patch test checks for a delayed type of allergic reaction, which occurs two to seven days after exposure. A positive test result produces contact dermatitis. This test is useful for diagnosing metal, latex, or chemical allergies.
When the results of a skin prick test are negative and the test will be used to make an important decision, an intradermal test may be used to either confirm the negative result or to more powerfully provoke a skin reaction. Examples of important decisions that might warrant intradermal skin testing include the selection of insect venom that will be included in allergy shots after a severe reaction, or the decision to use an antibiotic that had been previously blamed for an allergy reaction.
With the intradermal test, a small amount of allergen is injected directly into the skin rather than being scratched on the surface. This test is effective for diagnosing drug and insect allergies. It is never recommended for food allergy testing, because injection of food allergen may result in a severe allergic reaction. Moreover, this type of test for food allergy frequently produces false positive test results. As with the skin prick test, an observation period of at least 30 minutes is necessary after the test because of the potential for a serious reaction.
The radioallergosorbent test (RAST) requires a blood sample. The blood is tested in the laboratory to see if it contains IgE (immunoglobulin E) antibodies that attach to the surface of one or more common potential allergens (your blood is tested against a group of common allergens). If you have IgE antibodies that attach to an allergen’s surface, this is a sign of allergy to that substance.
This test is completely safe since you are not personally exposed to any allergy trigger (no observation period is necessary), but it is less accurate than skin tests. Very commonly, the RAST test will have a false negative result if your allergy has not been recently stimulated. Sometimes, you can also have a false positive reaction. In that case you have IgE antibodies to an allergen, yet you will not have an allergic reaction to that allergen.
Food Elimination Trial
Sometimes food allergies can be detected with a skin prick test, but the results of this test are not always reliable. For this reason, a food elimination trial is often used to confirm a food allergy suggested by the skin prick test.
In a food elimination trial, suspected food items are eliminated from your diet one at a time to see if their absence causes your symptoms to resolve. Then, if you feel better when a suspected food is eliminated, your health-care provider might add that food back to your diet to see if your symptoms recur. Recurring symptoms would confirm the allergy.
If you suspect a food allergy, you should not purposefully eat a trigger food to confirm your allergy unless you are being observed by a health care provider. Some types of foods, such as peanuts and shellfish, can cause anaphylaxis, which may be fatal. A rarely used test called the food challenge test requires that you swallow a pill containing allergens. This test is rarely done because it must be done in a hospital setting to monitor for a serious reaction.
Tests For Recurrent Hives
People with recurrent hives (urticaria) may be tested for several nonallergic conditions that can cause hives. These include liver or thyroid disease, vascular problems (called angioedema), lupus and infection (particularly fungal infections). Blood tests and a careful physical examination can check for these conditions. These problems cause hives far less often than allergic reactions do. These tests are only important for hives that have no obvious allergy trigger and are chronic.