What Is a Patch Test?
A patch test is a safe and painless investigative tool to confirm allergic contact dermatitis. Contact dermatitis is a kind of eczema triggered by direct skin contact with a particular substance. However, the use of several ingredients in cosmetics and the absence of labeling the ingredients in most cosmetics limit the use of patch tests. It also helps in the diagnosis of photoallergic dermatitis. The use of patch testing has also extended into research for investigating the irritant potential of various topical cosmetics like Balsam of Peru, formaldehyde, fragrance, and PPD (para-phenylenediamine). Patch tests can also identify slow, delayed allergic reactions, which can take several days to develop. Patch testing is usually done in a doctor's office. However, it can also be performed at home before embarking on skincare or cosmetic products. This is different from other allergy tests, like the skin prick test.
Why Is the Patch Test Done?
Patch testing is commonly used to diagnose allergies to substances such as:
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Metals (e.g., nickel, chromium).
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Fragrances.
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Preservatives.
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Rubber chemicals.
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Cosmetics.
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Medications.
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Plants (e.g., poison ivy, poison oak).
When Should a Patch Test Be Performed?
A patch testing is indicated for the following:
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Individuals with suspected allergic contact dermatitis to cosmetics.
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All cases of facial dermatitis, hands, and feet, which do not clear up in a short time.
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Persistent or atypical form of nummular eczema (coin-shaped skin rash).
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Skin care products may have many different ingredients that can irritate a person's skin or may cause an allergic reaction. A patch test can help a person check how their skin tolerates a product before using it over a larger area.
What Are the Different Types of Patch Tests?
Different types of patch tests are:
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Closed or Standard Patch Testing: In this test, offending cosmetics should be taken in appropriate amounts. It should be applied onto the common area of the skin, usually the upper back, and allowed to dry to facilitate evaporation of volatile contents and then occluded for 48 hours. The reading is taken after 72 hours. Positive results present as redness, swelling, and multiple solid small tiny raised lesions to fluid-filled eruptions at the site of contact.
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Indian Standard Series: Indian standard series contains mainly plant, cosmetics, and footwear series allergens.
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TRUE Test: TRUE stands for thin-layer rapid use epicutaneous test. The advantage of this kit is that the allergen is available in ready-to-use form.
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Chemotechnique Diagnostics Patch Test: Chemotechnique diagnostics patch test kit has tape-mounted polyethylene plastic chambers with more than 25 different series, including international standard series.
How to Perform a Patch Test?
Several allergens are applied to the back on special tapes fitted with small aluminum discs that help hold the individual allergens. Between 30 and over a hundred allergens can be applied at a time. Ideally, the patch tests are done when the dermatitis is dormant, but if the back skin is inflamed, the arms or abdominal skin can be used for application. After carefully marking the position of the patch test panels, they are removed after 48 hours, and the skin is further inspected 48 hours after that.
How to Prepare for a Patch Test?
Getting the skin ready for a patch test is necessary. Following these instructions can help an individual to get more precise, reliable results.
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Take less than 10 milligram of oral Prednisolone a day.
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Avoid topical steroids at the application site for three days before patch testing.
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Avoid taking an immunosuppressive agent that may reduce the reaction.
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Avoid extreme exercise that may dislodge the allergen tapes.
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Antihistamines can be taken as usual as these drugs do not interfere with the type IV hypersensitivity reaction.
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Before scheduling a patch test, inform the doctor of the current over-the-counter and prescription medications. Some drugs can interfere with allergic reactions, preventing patch testing from delivering accurate results.
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Other medications that can impede skin tests include:
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Certain heartburn medications, like Cimetidine and Ranitidine.
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Diphenhydramine, Cetirizine, and Fexofenadine.
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Omalizumab (a medication used to treat nasal polyps, asthma, and urticaria).
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Tricyclic antidepressants, such as Nortriptyline and Desipramine.
What to Avoid After the Patch Test?
After patch testing, it is best to:
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Avoid scratching, as itching indicates a positive reaction, and scratching might alter the test results.
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Keep the application area dry until the patches are removed in two to three days. One may take a sponge bath, but avoid getting the back wet.
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Avoid strenuous exercises that can cause excessive perspiration and detachment of the test unit.
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If patches peel or loosen from the skin, ask someone to apply pressure to the adhesive portion of the strips. If necessary, apply additional tape to the edges of the chamber units.
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Avoid too much sun exposure to the application area.
What Are the Potential Side Effects of Patch Tests?
The potential adverse reactions include:
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Post-inflammatory pigmentation.
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Exacerbation of existing contact allergic dermatitis and persistent positive reaction for more than one month.
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Active sensitization to a new allergen to which an individual was not exposed in the past and got sensitivity after the patch testing procedure.
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Contact urticaria may spread beyond the site of application.
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Irritant contact dermatitis because of adhesive in the plaster.
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Change in color at the procedure site and rarely scarring or keloid formation.
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Rarely, applying the allergen may cause sensitization during the patch testing, producing a positive reaction about 10 to 14 days after the patches are applied.
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Multiple positive patch tests may arise if the dermatitis is very active during patch testing, making it difficult to interpret. Re-patch testing may be needed when the eczema is dormant.
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Serious side effects rarely develop from patch testing. But for a few people, an infection or life-threatening allergic reaction can occur within 30 minutes of applying the patches. Signs of these life-threatening allergic reactions include nausea, vomiting, swelling, and difficulty breathing. If someone sees these signs, seek immediate medical treatment.
Conclusion:
A patch test is a common investigative tool to confirm allergic contact dermatitis. Closed or standard patch testing, Indian standard series, TRUE Test, and chemotechnique diagnostics patch test are the different types of patch tests. The patches can cause skin irritation in some people. But it usually clears on its own in a few days. To get accurate results, always follow the doctor's instructions.
