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
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.
Because antihistamines interfere with skin testing, you must refrain from using any antihistamines for 5 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.
Listed below are some common antihistamines or medications containing antihistamines. The list is by no means complete or comprehensive, and some medications such as certain antidepressants like Sinequan may have strong anti-histamine effects. If you are taking something that is not listed below and are unsure if it contains an antihistamine, contact your pharmacist or doctor. If you believe your symptoms are too severe to stop your antihistamine, you may still be evaluated, although skin testing may be
Prescription Antihistamines:
Oral—Allegra & Allegra-D; Atarax (hydroxyzine); Benadryl; Claritin & Claritin-D; Dimetapp; Phenergan; Tavist & Tavist-D; Zyrtec; Bromfed; Polyhistine;Rynatan; Semprex-D
Nasal—Astelin;
Robitussin--Cold Night Time Liquigels & CF; Benadryl Allergy; Nyquil; Drixoral Cold & Allergy; Alka-Seltzer Plus Products; Extra Strength Bayer PM; Midol Menstrual Pain & Multi-Symptom Menstrual Relief; Midol PMS; Nytol; Comtrex; Excedrin PM; Tylenol-- Multi-Symptom Formula Tylenol Cold, Maximum Strength Tylenol Flu Night Time Medicine, Tylenol PM, Tylenol Severe Allergy, Maximum Strength Tylenol Allergy Sinus Night Time, Maximum Strength Tylenol Allergy Sinus; Theraflu; Unisom Products; Sominex; Contac brand products
RAST testing uses a sample of the patient’s blood to look for evidence of allergy. While this avoids the need for stopping antihistamines, RAST tests are more expensive, take longer, and are less sensitive than skin tests. They are generally reserved for patients who cannot be skin tested. RAST tests may also serve a useful role where skin testing reagents are not available, such as evaluating suspected latex allergy, or where skin testing is particularly cumbersome, as with allergy to certain insects, such as “bees”.