Skin Patch and Skin Prick Allergy Testing

This type of testing is not suitable to everyone, your doctor will decide which type of allergy test to administer. If your skin is very sensitive or reactive, covered in eczema or you react aggressively to your allergens, a RAST may be recommended as an alternative.

 Patch Testing

This is used to evaluate for an underlying trigger/agent in individuals who develop contact dermatitis. Examples of contact dermatitis is the development of a rash after wearing certain metal jewelry or using a certain skin care product. The suspected agents are placed in the form of patches on the back. The patches are removed 48 hours later. The results are read 2 to 5 days after the patches are removed, and sometimes up to 10 days after removal. 

 Allergy Skin Testing

This is used to evaluate for suspected environmental or seasonal allergies, food allergy, stinging insect allergy, and certain drug/medication allergies. During skin testing the suspected allergen is placed on the skin and the test results are read after 15 minutes. An allergen is a substance that can cause an allergic reaction. There are 2 methods for skin testing: prick skin testing and intradermal skin testing.

Prick skin testing: Individual solutions containing the suspected allergens are placed on the skin using a prick device. The test results are available 15 minutes later.

Intradermal skin testing: Individual solutions containing the suspected allergens are placed just underneath the surface of the skin using a small needle. The test results are available 15 minutes later.

Your doctor will determine which specific type of skin test is required and will interpret the results of the tests.

The skin scratch test, another testing modality, is not recommended as a test for inhalant or food allergens since results are more difficult to interpret and standardize. Scratch testing may result in varying quantities of allergen absorbed, mechanical irritation of the skin , bleeding at the test site, and carries a higher risk of inducing a systemic allergic reaction.

The Procedure for Patch Testing

A patch test is a skin test used to find the cause of a possible allergic reaction on the skin. This reaction is called allergic contact dermatitis. Contact dermatitis is a reaction to something that came into contact with the skin. This kind of allergic reaction usually causes inflammation (redness, itching). 

It is important to follow your doctors guidelines to get accurate results. Patch testing evaluates a slow, delayed skin reaction so it is important that the patches stay in place. The test site must also be protected throughout the entire test week. 

Preparation for the Test

Getting the skin ready for your patch test is important. Following these instructions will help you to get more accurate, reliable results. 

• Avoid sun exposure for 1 to 2 weeks before patch testing.
• Do not use topical medicines (creams and ointments) on the back and any other area where patches may be placed for at least 1 week before patch testing. You can continue to use these medicines on areas of the body where patches will not be placed.
• You may use moisturizers on the skin until the day before patch testing.
• You may continue to take any prescribed antihistamines as usual before and during the testing.

During the Test Week

• The patches and test area marked with ink must be kept dry throughout the entire test.
• Do not apply anything to the test area. This includes all soaps, creams, ointments and moisturizers.
• Do not scratch, rub, loosen or remove the patches.
• Avoid physical activities that will loosen the tape.
• Do not expose the test area to sunlight.
• Avoid hot areas and activities that cause excessive sweating.

Continue to take your prescribed antihistamines. 

The patches must be kept dry. You should not take a shower or bath. Wash areas of the body where there are no patches with a washcloth or bath sponge. Be careful not to get the patches wet. 

The First Visit:

The patches are usually placed on your back and left in place for 2 days. 

• You should wear loose, comfortable clothing.
• You should wear old, dark-colored clothes to avoid stains from the marker.

Small sheets, or patches, holding different substances are placed on the skin. The nurses will use a marker to outline the edges of the sheets. Then the sheets will be secured with tape or another sticky bandage (dressing). 

The Second Visit:

The second visit is usually 2 days after the patches were put on. Typically you will come to the clinic to have the patches removed. The nurses will carefully remove the tape and patches. They will re-mark the skin where the patches were. The nurses will check the area to see if the skin shows any signs of a reaction. In many cases a reaction may not have shown up yet. It is still important to come back to the clinic to have the area checked one more time. 

Continue to keep the skin dry where the patches were. Do not take a shower or bathe where the patches were. Do not apply creams, ointments, or moisturizers where the patches were. Do not scratch the skin where the patches were. You should avoid hot areas and activities that cause excessive sweating. 

Removing patches: Normally you should not remove patches yourself. Usually the nurses in the dermatology clinic remove them. Occasionally the doctor will ask a family to remove the patches at home. If you have been given this instruction you will remove the tape and clear bandage carefully. You will re-mark the borders of the patch testing sheets with the marker you were given before you take them off the skin. 

Sometimes a patch may fall off or pull away so that it is not actually touching the skin. If this happens it is important to let your doctor know at your next appointment. 

The Third Visit: 

The third visit is usually 4 to 5 days after the patches were put on. You will come to the clinic to have the skin checked. It is important to check the skin again to look for skin reactions. The nurses and doctor will look at your skin and tell you if there are any substances that you should avoid. 

Which Allergens are tested

This will broadly be determined by where you live. Different countries have different plants and environmental allergens that will be included in the test. The top allergens from 2005–06 in the US were: 

• nickel sulfate (19.0%),
• Myroxylon pereirae (Balsam of Peru, 11.9%),
• fragrance mix I (11.5%),
• quaternium-15 (10.3%),
• neomycin (10.0%),
• bacitracin (9.2%),
• formaldehyde (9.0%),
• cobalt chloride (8.4%),
• methyldibromoglutaronitrile/phenoxyethanol (5.8%),
• p-phenylenediamine (5.0%),
• potassium dichromate (4.8%),
• carba mix (3.9%),
• thiuram mix (3.9%),
• diazolidinyl urea (3.7%),
• 2-bromo-2-nitropropane-1,3-diol (3.4%).

The most frequent allergen recorded in many research studies around the world is nickel. Nickel allergy is more prevalent in young women, and is especially associated with ear piercing or any nickel-containing watch, belt, zipper, or jewelry. Other common allergens are surveyed in North America by the North American Contact Dermatitis Group (NACDG).