A skin prick test, also called a puncture or scratch test, checks for immediate allergic reactions to as many as 40 different substances at once. This test is usually done to identify allergies to pollen, mold, pet dander, dust mites, and foods.
In adults, the test is usually done on the forearm. Children may be tested on the upper back.
Allergy skin tests aren’t painful. This type of testing uses needles (lancets) that barely penetrate the skin’s surface. You won’t bleed or feel more than mild, momentary discomfort.
After cleaning the test site with alcohol, the nurse draws small marks on your skin and applies a drop of allergen extract next to each mark. He or she then uses a lancet to prick the extracts into the skin’s surface. A new lancet is used for each allergen.
To see if your skin is reacting normally, two additional substances are scratched into your skin’s surface:
- Histamine. In most people, this substance causes a skin response. If you don’t react to histamine, your allergy skin test may not reveal an allergy even if you have one.
- Glycerin or saline. In most people, these substances don’t cause any reaction. If you do react to glycerin or saline, you may have sensitive skin. Test results will need to be interpreted cautiously to avoid a false allergy diagnosis.
About 15 minutes after the skin pricks, the nurse observes your skin for signs of allergic reactions.
If you are allergic to one of the substances tested, you’ll develop a raised, red, itchy bump (wheal) that may look like a mosquito bite. A nurse will then measure the bump’s size.
After the nurse records the results, he or she will clean your skin with alcohol to remove the marks.
You may need a test that uses a needle to inject a small amount of allergen extract just into the skin on your arm (intradermal test).
The injection site is examined after about 15 minutes for signs of an allergic reaction. Your doctor may recommend this test to check for an allergy to insect venom or penicillin.
Patch testing is generally done to see whether a particular substance is causing allergic skin irritation (contact dermatitis). Patch tests can detect delayed allergic reactions, which can take several days to develop.
Patch tests don’t use needles. Instead, allergens are applied to patches, which are then placed on your skin. During a patch test, your skin may be exposed to 20 to 30 extracts of substances that can cause contact dermatitis.
These can include latex, medications, fragrances, preservatives, hair dyes, metals, and resins.
You wear the patches on your arm or back for 48 hours. During this time, you should avoid bathing and activities that cause heavy sweating.
The patches are removed when you return to your doctor’s office. Irritated skin at the patch site may indicate an allergy.
Before recommending a skin test, your doctor will ask you detailed questions about your medical history, your signs and symptoms, and your usual way of treating them.
Your answers can help your doctor determine if allergies run in your family and if an allergic reaction is most likely causing your symptoms. Your doctor may also perform a physical examination to search for additional clues about the cause of your signs and symptoms.
Medications can interfere with results
Before scheduling a skin test, bring your doctor a list of all of your prescription and over-the-counter medications. Some medications can suppress allergic reactions, preventing skin testing from giving accurate results.
Other medications may increase your risk of developing a severe allergic reaction during a test.
Because medications clear out of your system at different rates, your doctor may ask that you stop taking certain medications for up to 10 days.
Medications that can interfere with skin tests include:
- Prescription antihistamines,such as levocetirizine (Xyzal) and desloratadine (Clarinex).
- Over-the-counter antihistamines,such as loratadine (Claritin, Alavert), diphenhydramine (Benadryl, others), chlorpheniramine (Chlor-Trimeton), cetirizine (Zyrtec, others) and fexofenadine (Allegra).
- Tricyclic antidepressants, such as nortriptyline (Pamelor) and desipramine (Norpramin).
- Certain heartburn medications, such as cimetidine (Tagamet) and ranitidine (Zantac).
- The asthma medication is omalizumab (Xolair). This medication can disrupt test results for six months or longer even after you quit using it (most medications affect results for days to weeks).