Patch Test
The patch test is a technique used to identify which substances trigger a skin allergic reaction in the patient. It involves applying various allergens (substances that may cause an allergic reaction) to a dressing that is placed on the patient's skin for several days.
General Description
The patch test is a type of epicutaneous allergy test used to identify the specific substances causing an allergic reaction in the patient, known as contact dermatitis.
Contact dermatitis is an inflammatory response of the immune system that manifests on the skin when it comes into contact with certain allergens or irritants. Common irritants include detergents, soaps, solvents, metals, cosmetic ingredients, rubber, or plants.
When is it indicated?
The patch test is indicated for patients who exhibit typical symptoms of contact dermatitis on the skin, including:
- Redness and inflammation.
- Spots.
- Dry, cracked, and scaly skin.
- Rash, blisters, or scabs, sometimes with exudate.
- Itching, burning.
- Increased sensitivity.
How is it performed?
The basis of the test is to expose the patient's skin, in a controlled manner, to allergens that may cause the reaction. Polyethylene patches are used for this purpose, which are impregnated with different allergenic extracts, meaning aqueous solutions made from biological compounds derived from natural allergens. Each patch is also numbered to identify the substance applied to it. These patches are placed on the patient's back (at least three centimeters apart from each other) and are covered with adhesive paper, ensuring continuous contact with the skin for 48 hours.
The number of patches used depends on the number of suspected substances to be tested. It is common to use a standard selection of allergens, such as the battery determined by the Spanish Group for Research in Contact Dermatitis and Cutaneous Immunoallergy (GEIDAC), which consists of about 30 allergens. Additionally, specific tests can be conducted with substances that the patient comes into daily contact with.
After 48 hours, the patches are removed and a first reading of the results is performed, meaning it is checked which substances have caused an allergic reaction (eczemaEczemaEczema , redness, swelling). After 96 hours, without washing the skin in the area, a second reading is performed to obtain the definitive diagnosis. A third reading, six or seven days after the patches are applied, is also common to detect any delayed reactions.
Risks
Occasionally, a condition known as the excited skin syndrome may appear, meaning that the positive reaction to one of the allergenic substances in the patch is so strong that it also manifests in other areas covered by other patches, leading to false positives. It is also possible, though not common, that sensitization to one of the substances applied may occur. Additionally, spots or discolorations may appear on the skin, which can take some time to disappear.
What to expect from a patch test
Before beginning, the patient should remove the clothing from the upper part of the body. The patches are placed on healthy skin, preferably on the upper back. If there are rashes or lesions, they are applied to the outer part of the arm. The application of the patches may take more or less time depending on the number of substances being tested, but it typically does not exceed 40 minutes.
While wearing the patches and until the last reading, the area should remain completely dry and sweat-free, as the patches may shift or detach, and some substances may be eliminated. Therefore, the area should not be washed or subjected to intense physical activity. Additionally, the skin should not be exposed to sunlight, as some substances may react to it. If any patch detaches, it should be repositioned immediately and secured with adhesive tape.
It is normal to feel itching or burning in the skin, but the patient should avoid scratching, as this could irritate the skin and lead to incorrect interpretation of the results. If the results are positive, the reaction caused may last for several weeks.
Specialties requesting the patch test
The patch test is requested in the allergy unit.
How to prepare
Before taking the test, the skin should be cleaned, but without applying creams, ointments, or lotions. If there is a lot of body hair, shaving it the day before may be necessary. Additionally, sun exposure should be avoided during the two weeks prior to the test, as it reduces the skin's reactivity, as well as the discontinuation of systemic or topical corticosteroids and antihistamines, as these medications can influence the results of the patch test.
