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Patch test: contact test is important in the diagnosis of contact dermatitis

Patch test: contact test is important in the diagnosis of contact dermatitis

The patch test also called the epicutaneous test or contact test is considered the gold standard in the diagnosis of allergic contact dermatitis. Contact dermatitis is defined as an inflammatory response in the skin resulting from topical exposure of external substances.

Clinically, contact dermatitis is characterized by the appearance of an eczema located in the region of the body in contact with external agent. In an acute phase it can cause the appearance of itchy skin, erythema (redness), edema (swelling) and vesicles (small blisters). In a subacute phase there is a rupture of the vesicles with formation of crusts and in a chronic phase there is a process called lichenification (the skin becomes very dry with desquamation and with lines of depression). Substances that trigger contact dermatitis may be present at work (occupational contact dermatitis), it can be a cosmetic, a medicine, a plant or a gift in leisure activities.

Contact dermatitis occurs through the action of substances on the skin that functions as primary irritants (non-allergic mechanisms involved) or substances that cause lesions by hypersensitivity mechanisms (allergic mechanisms are involved).

Examples of contact dermatitis:

  • Face: nail polish, perfumes,

  • - Eyelid: nail polish, eyelashes, eye drops, cosmetics

  • - Neck: perfume (for example, hair lotions, powdered materials, paints and varnishes), moisturizers, sunscreens and cosmetics , necklaces, necklaces, leather coats

  • - Pectoral region: necklaces, bras, soaps

  • - Axilla: perfumes, powders, deodorant and depilatory cream

  • - Hands: gloves, rings, coins, automobile steering wheel

  • - Related to the professions: jewelers (nickel, chrome, leather, gold), medical (rubber, latex), footwear (leather, rubber or plastic), socks, carpenters (pine sawdust, tar), cooks (flavorings, condiments), cobblers (rubber, epoxy resin).

  • How the test is performed

The contact test can only be done by an allergist and is carried out directly on the skin of the patient being considered a "in vivo" biological test. It consists of placing the specific substances in contact with the patient's skin, causing exposure of the patient to the allergen and producing areas of dermatitis, that is, it functions as a provocation test to the contactor. It is performed in three steps: at first, 48 hours later and 96 hours later. At the end of the 96 hours it is possible to provide a report.

The Brazilian standard contact test battery is composed of 30 substances standardized by the specialized dermatological allergy department of the Brazilian Society of Dermatology. There are complementary batteries that can be used on a patient's need (cosmetics, professionals).

Contact test steps

1) The 30 substances (mostly semi-solid consistency) are applied in con- tainers in rectangular format with paper, aluminum or plastic cameras on which the test battery substances are placed). Each container contains 10 cameras.

2) The patient's skin should be defatted with cotton soaked with alcohol or ether on the back (back).

3) Containers with the substances applied and left on the patient's back for 48 hours. In these 48 hours the patient will not be able to wet the area and should avoid doing physical exercises like aerobics and swimming.4) After 48 hours the test is removed and the first reading is performed after 30 minutes of removal of the adhesives, using a special ruler.

5) The location where the con- tainers were is identified for the next reading of 96

6) Between the reading of 48 and 96 hours it is allowed to wet the back without the use of soaps on the spot.

7) The second reading is performed after 96 hours of the test with the aid of a ruler

8) The result of the test is based on the following interpretation of the specialized dermatological allergy department of the Brazilian Society of Dermatology

(-) Negative

(?) Doubtful - slight erythema (redness)

(+) Positive weak - erythema and papules (elevation forming small plaque)

(++) Strong positive - erythema, papules and vesicles (small blisters)

papules and confluent vesicles can form bubbles

9) When the contact test is negative the substance involved is likely to be a primary irritant since dermatitis occurs without the involvement of an allergic mechanism, or it is another disease, or there is another substance that has not been tested on this battery. When the contact test is positive it indicates allergic contact dermatitis.

Care to be taken before the test

1. The test should not be performed in the presence of active (acute) lesions.

2. Antihistamines do not interfere with the test result.

3. Systemic corticosteroids (oral) should be discontinued, injectable depot corticosteroids may interfere in the outcome for 30 days. The use of topical corticosteroids at the test site should be avoided for 15 days.

4. The patient should not be subjected to sun exposure 15 days before the test.

5. Immunosuppressive drugs (azathioprine) may negate the test. 6. It is recommended to avoid carrying out the test during pregnancy.

Fotopatch test

It is a variation of the contact test for photosensitizing substances (sensitivity to sunlight). The technique used is the same, but the substances are tested in duplicate (instead of 3 adhesives 6 are used) on both sides of the patient's back. After 48 hours the tests are removed and the first reading is performed. Then one side is covered with opaque material and the other side is irradiated with ultraviolet radiation (through a special lamp). The second reading is performed 96 hours after the placement of the test, comparing the irradiated site with the non-irradiated site. The positive test will show lesions at the irradiated site.

Example of photoallergic contact dermatitis: perfumes, topical antihistamine (patient uses ointment and is exposed to sunlight, develops dermatitis by sunlight), topical antifungal , non-steroidal anti-inflammatory drugs.

Once the cause of the allergy has been identified, the patient should avoid contact with it. The specialist doctor will indicate alternatives to your protection to improve your quality of life.


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