Allergy Skin Tests | Scratch or Prick Test | Intradermal Test

SKIN TESTS

Skin tests are the most popular diagnostic tool, and according to the Journal of the American Medical Association (JAMA), they're "the least time-consuming and expensive" and the "most re­vealing tests for disclosing specific sensitivities."

The American Academy of Allergy, Asthma and Immunol­ogy (AAAAI), the nation's largest professional medical society representing allergists, immunologists, and related profession­als, agrees: "[Skin tests] are quicker than RAST, less expensive, and better standardized."

Research is ongoing at the Food and Drug Administration (FDA), but because of limited funds and higher priorities, only a few extracts of dust and pollen have been standardized for use in tests and treatment. The advantages of stan­dardization are that you can count on safe, high-quality prod­ucts, and you can change doctors without having to change injection dosage.

There's no way to prepare for skin tests, but you'll be asked to stop certain medications, especially antihistamines, a week or so beforehand. Some of the new medications stay even longer in your system, so be sure the doctor knows exactly what you've been taking.

Three types of skin test are available. Some doctors use only one method; others combine them.

Scratch or Prick Test

A technician places dilute drops of suspected allergens on the skin of the upper arm or back, then scratches them lightly with a sterile needle. (Some technicians use a boardlike device that they press to the skin to apply eight extracts at once.)

After ten to thirty minutes (allergists use different dilutions that have slightly varying time intervals), a reddish wheal, like a mosquito bite, may appear. Its size indicates the degree of sen­sitivity to the allergen. Reactions can also occur several hours later, or even the next day.

Intradermal Test

Minute amounts of allergen are injected just below the surface of the skin. You feel only a tiny sting. Again, you'll wait ten to thirty minutes for the response to be measured.

Intradermal tests are more sensitive than scratch tests and are sometimes used to confirm scratch test results, especially for stinging insects and drugs.

Patch Test

Small absorbent pads containing allergen are affixed to the skin with nonallergenic tape, left there for twenty-four to seventy-two hours, then evaluated. You'll be asked not to bathe that area or engage in vigorous physical activity. If you have severe pain or itching, remove the patch immediately and call your doctor.

Patch tests are particularly helpful in diagnosing allergic contact dermatitis (ACD), a rash that comes from direct skin contact. Some critics say skin tests are too sensitive and give positive readings where no allergy exists. Also, the readings are not reliable for infants and people over sixty. Nevertheless, skin tests have proven to be relatively accurate for most substances except foods and are generally safe. The major risk with any in vivo allergy test is an anaphylactic reaction, but it's rare.