Skin allergy testing is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.
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A microscopic amount of an allergen is introduced to a patient's skin by various means:
If an immuno-response is seen in the form of a rash, urticaria (hives), or (worse) anaphylaxis it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen. Further testing can be done to identify the particular allergen.
The "scratch test" as it's called, is still very commonly used as an allergen test. A similar test involving injecting the allergen is also used, but is not quite as common due to increased likelihood of infection and general ineffectiveness by comparison. There are other methods available to test for allergy.
Some allergies are identified in a few minutes but others may take several days. In all cases where the test is positive, the skin will become raised, red and appear itchy. The results are recorded- larger wheals indicating that the subject is more sensitive to that particular allergen. A negative test does not mean that the subject is not allergic; simply that either the right concentration was not used or the body failed to elicit a response.
In the prick (scratch) test, a few drops of the purified allergen are gently pricked on to the skin surface, usually the forearm. This test is usually done in order to identify allergies to pet dander, dust, pollen, foods or dust mites. Intradermal injections are done by injecting a small amount of allergen just beneath the skin surface. The test is done to assess allergies to drugs like penicillin or bee venom.
To ensure that the skin is reacting in the way it is supposed to, all skin allergy tests are also performed with proven allergens like histamine or glycerin. The majority of people do react to histamine or glycerin. If the skin does not react to these allergens then it most likely will not react to the other allergens. These results are interpreted as falsely negative.[1]
The patch test simply uses a large patch which has different allergens on it. The patch is applied onto the skin, usually on the back. The allergens on the patch include latex, medications, preservatives, hair dyes, fragrances, resins and various metals. When a patch is applied the subject should avoid bathing or exercise for at least 48 hours.
Also called an intra dermal test, this skin end point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response.[2] To prevent a severe allergic reaction, the test is started with a very dilute solution. After 10 minutes, the injection site is measured to look for growth of wheal, a small swelling of the skin. Two millimeters of growth in 10 minutes is considered positive. If 2 mm of growth is noted, then a second injection at a higher concentration is given to confirm the response. The end point is the concentration of antigen that causes an increase in the size of the wheal followed by confirmatory whealing. If the wheal grows larger than 13 mm, then no further injection are given since this is considered a major reaction.
There are no major preparations required for skin testing. At the first consult, the subject's medical history is obtained and physical examination is performed. All consumers should bring a list of their medications because some may interfere with the testing. Other medications may increase the chance of a severe allergic reaction. Medications that commonly interfere with skin testing include the following:
Consumers who undergo skin testing should know that anaphylaxis can occur anytime. So if any of the following symptoms are experienced, a physician consultation is recommended immediately:
Even though skin testing may sound like a benign procedure it does have some risks which include swollen red bumps (hives) may occur after the test. The hives usually disappear in a few hours after the test. In rare cases they can persist for a day or two. These hives may be itchy and are best treated by applying an over the counter hydrocortisone cream.[3] In very rare cases one may develop a full blown allergic reaction. Physicians who perform skin test always have equipment and medications available in case an anaphylaxis reaction occurs. This is the main reason why consumers should not get skin testing performed at corner stores or by people who have no medical training.[4]
Skin testing can be done on individuals of all ages. However, there are times when a skin test should not be done. Individuals who take medications for depression, gastric acidity or antihistamines should not undergo this test. In such cases, stopping the medications for a skin test may not be worthwhile as one may develop symptoms from the untreated medical disorders. Individuals who have severe, generalized skin disease or an acute skin infection should not undergo skin testing. One needs uninvolved skin for testing.
There are some individuals who are highly sensitive to even the smallest amount of allergen and in such scenarios, allergic testing is not recommended. Whenever the chances of an anaphylactic shock are high, the test is best avoided.[5]
Besides skin tests, there are blood tests which measure a specific antibody in the blood. The IgE antibody plays a vital role in allergies but its levels in blood do not always correlate with the allergic reaction.[6]
There are many alternative health care practitioners who perform a variety of provocation neutralization tests, but the vast majority of these tests have no validity and have never been proven to work scientifically.
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