Skin testing is the most reliable form of allergy testing. Because mast cells are located in high numbers just under the skin, results of skin testing has proven to be more accurate than blood testing in diagnosing allergies.
The Skin Test Procedure
The surface of your skin is "scratched" with minute amounts of a number of specific allergens like mold spores, pollen, dust mites, and/or animal dander. When the results are positive, a small reaction on the skin occurs, usually within 20 minutes. A small bump may appear and there may be some itchiness. This indicates that you allergic to that specific trigger. If no bump appears, you do not have a reaction to that specific allergen.
Depending on the results of your skin test, your allergist will determine the best way to treat your allergy symptoms.
SKIN PRICK TESTING
Skin prick testing for allergies is generally done on the patient’s back, but may be done on the arm under special circumstances. The skin on the back or arm is first wiped down with alcohol. Next, the nurse uses a marker to write down where he or she will place the allergens. One drop of each allergen is placed on the skin. A small needle is then placed through each of the drops to irritate the top layer of skin. After 15 to 20 minutes, the reactions on the patient’s back are measured and compared to saline (salt water, to see how the patient reacts to just the “prick”) and histamine (to make sure the patient hasn’t accidentally taken an antihistamine and to see how reactive they are). Prick testing generally causes minimal discomfort. Serious reactions are extremely rare although occasionally patients may have a more prolonged spot at the site of strong responses.
Intradermal testing is done on the patient’s upper arms. This may be done for one of the following reasons:
1. No evidence of allergy appeared with prick tests.
2. Some evidence of allergy appeared with prick tests but further evaluation is necessary.
Intradermal tests are performed by injecting a small amount of fluid into the top layer of skin with a small needle. This will produce a small “bubble”. After 15-20 minutes, the reactions are measured. There is a bit more discomfort with intradermal testing, but they are also usually well tolerated. Again, serious reactions are very rare, but there may be a sustained area of local redness.
MEDICATIONS AND SKIN TESTING
Because antihistamines interfere with skin testing, you must refrain from using any antihistamines for 2 days prior to being tested. Patients also should be off all “beta-blocker” medications. These drugs are used to treat high blood pressure, irregular heartbeats, heart failure, coronary artery disease, tremor and glaucoma. Patients should not stop their beta-blocker therapy without consulting their physician or the doctor who prescribed the medication.