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Skin testing for allergies is one of several ways the medical profession can diagnose what allergens a patient is suffering from. Skin tests include the skin prick test, intradermal test and patch test. Other ways to test for allergies include blood tests, elimination testing, provocation testing and hair analysis.
An allergy skin test is carried out in a medical clinic by trained personnel, usually a doctor or nurse. For the basic skin prick or scratch test, the test site is cleaned with alcohol before it is marked with rows of dots with a special dye, so the spot where each allergen is applied isn’t lost. Each allergen is applied with a new lancet next to a dot. The instrument hardly breaks the skin’s surface. The preferred site is the inner forearm for adults and the upper back for children.
Many dozen substances can be tested at once, although the number will be lower if the patient has been able to narrow down the possible causes of their allergy. After 15 to 30 minutes, the nurse inspects the site for allergic reactions. A positive reaction to a substance will show up as a red mark for a mild reaction through to a larger hive or wheal for a more severe reaction.
The nurse then tests the skin for two more substances: histamine and glycerin or saline, to see if the skin is reacting to the allergens properly. Histamine causes a reaction in nearly everyone’s skin and is used as the positive control. Glycerin or saline reacts in almost no one, so it is used as the negative control. The use of these substances helps provide an indication of the accuracy of the test.
Sometimes a skin prick test is inconclusive and an intradermal test in conducted where allergens are injected into the arm. This is common if a person is thought to be allergic to penicillin or an insect bite.
A third type of test is the skin patch test where allergens are put on a pad and taped to the skin for 1 to 3 days. This test might be used to find the cause of contact dermatitis, such as latex, medication, preservative or hair dye.
Skin tests are more accurate for pollen, dust and insect bites than for food and medication. Food allergies are usually quite complex and skin test diagnosis is therefore less accurate. The only medication that can be satisfactorily skin tested is penicillin.
Further, the accuracy of tests varies from person to person. Some people test positive for a substance, yet they have outgrown the allergy or have never had a reaction to the substance before. Other people can test negative to a substance, yet they are sure they have an allergy to it. Further, test results can differ over time but the person doesn’t think their allergy is any different.
As it is not possible to be immunized for allergies, some people feel the tests are not worthwhile and simply avoid the things that cause them problems. That’s alright for those who have been able to determine the cause of their allergy. For other people, the results of a skin test are at least indicative of the problem and this is better than guessing. Some people see the results as a message – they see their skin swell up quite quickly due to a minute amount of the allergen and realize what a large amount might do, and are more careful to avoid the allergen.
There are a number of things a patient can do to increase the accuracy of a skin test for allergies. The first one is to discuss the allergy in detail with their doctor. This may include the patient’s medical history, lifestyle, diet, suspected allergies, other illnesses, symptoms, what they’ve done to treat the allergies, family history of allergies, and whether they’ve kept a food and allergy diary. If the allergen has been isolated, avoidance may be the answer and a skin test might not be necessary.
The doctor will also need to know the patient’s medication details. Some medications suppress allergies while others are likely to bring an allergy on, making skin testing less accurate. Antihistamines, antidepressants and heartburn medications can affect skin tests for allergies.
The accuracy and other aspects of allergy skin tests should be compared with alternative tests. Skin tests are regarded as safe, cheap, easy and usually reliable. The main alternative is blood tests, which search the blood for antibodies. An antibody has to be present for an allergic reaction to take place. Blood tests are less sensitive than skin tests and for this reason may be more suitable for some people. These tests are more likely to pick up food allergies than skin tests.
Elimination testing is another common test. Here, suspected foods are cut from the diet for a few weeks and then reintroduced one by one. Provocation testing is where allergens are introduced under controlled circumstances and should be supervised by a doctor. There is also hair testing, although this is not generally supported by the medical fraternity.
The accuracy of skin tests for allergies can depend on a number of factors, the most important being the type of substances a person is thought to be allergic to. Skin tests are more accurate in detecting airborne and insect allergies but less accurate for food and medication allergies. Further, the accuracy of skin tests varies from person to person.