Allergy Testing

By editor | October 16, 2007

A doctor needs to know exactly what your child is allergic to in order to treat allergies effectively. The best way to detect particular allergies is by taking a type of skin test called a prick or scratch test. Skin testing is the quickest and easiest way to test a child’s reaction to a large number of allergens all at once. By using this technique a doctor will be able to give you an answer usually within fifteen minutes.

To do a skin test, a doctor or nurse will place small drop of each different allergen on the arm or back. The drop is then scratched with a special fork or needle. Sometimes the drop and scratch are done at the same time. If a child is allergic to a certain substance, the skin of that scratch will react by swelling and turning red, usually within fifteen minutes or so. The reaction dies down again quickly, usually within another thirty minutes.

Skin testing may sound painful, but it hardly hurts at all. The special forks or needles that the doctor uses are very fine and barely penetrate the top layer of skin. The swelling from any reactions goes away quickly, although the area may feel itchy for a few hours. You can prepare your child for skin testing by explaining the procedure in advance and assuring him that the scratches will be shallow, the needles will be very thin, and it won’t hurt. In fact, it’s nothing like getting a shot.

Skin testing can be done safely on children of any age, even infants. If your child has been taking any antihistamines (medications to treat allergy symptoms), it’s important for him to stop taking them before the test or the results won’t be accurate. If your child takes antihistamines or any other medicines, discuss them with the doctor well in advance of the testing. Your child may have to stop taking the medication temporarily.

Dangerous reactions to skin testing are very rare, but they are possible, so the test must be done by a specialist in a medical office where the equipment and staff are available to treat a bad reaction. If the itching from the skin tests doesn’t go away after a few hours, or if a child is short of breath or wheezes after the test, he could be having a bad reaction to the skin testing. Call the doctor at once or go to the nearest emergency room.

Skin testing is a highly reliable way to identify allergies, but it’s not always completely accurate. Sometimes a child can have a strong allergic reaction to something but have a negative skin test for the same substance. Because there are many possible reasons for this, your allergist will probably want to do other tests.

If results of the scratch test are unclear, in some cases the allergist will inject a tiny amount of an allergen directly under the skin, usually on the upper arm, to see if there is a reaction. This sort of skin testing is a bit more uncomfortable, but the needle is hardly noticeable and the reaction, if there is one, is very mild.

Another type of allergy testing is called radioallergosorbent blood test, or RAST for short. This test uses a blood sample to check for different IgE antibodies. It’s more expensive than skin testing, and the results can take as long as two weeks to become available. RAST testing is generally used only when skin testing can’t be done for instance, if the child has extensive eczema, has to take a medicine every day, or has asthma that isn’t under control.

With all these tests, a false negative result may occur, which means allergic symptoms are evident, but the test is negative. The test results, therefore, have to be interpreted along with the history and physical exam that your physician had done previously.


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