Patch tests

What are patch tests?

Patch testing, also known as epicutaneous testing, is primarily a process by which you can find out if a certain substance is the culprit in causing inflammation when it comes into contact with the skin.

What does patch testing consist of?

First of all, you have to have a conversation with the specialist in which you explain when it started, its development, the treatments that have been tried, family history, your job…

The patch as such consists of an aluminum strip covered laterally by polyethylene. In the center, a kind of filter paper disc is placed, on which the substances to be investigated are applied. The patches are usually placed on the back, from the scapula downwards in strips. Activities involving sweating should be avoided, and the area should not be washed.

Once 48 hours have elapsed with the patches, they are removed, and the reading is taken one hour later. About 48 hours after patch removal, the final reading is taken.

Why are patch tests performed?

These tests are performed to recognize whether a certain substance is the cause of inflammation on the skin when it comes into contact with the skin. This inflammation is known as contact dermatitis. There are at least two types of contact dermatitis: irritant contact dermatitis, in which no immune mechanism is involved, and allergic contact dermatitis, in which the immune system mediates.

Thus, in cases in which a person suffers from a recent dermatitis or if he/she has a persistent and long-lasting eczema, the dermatologist may think that it is an allergic contact dermatitis. There are also cases in which the application of a drug for a period of time to a certain area of the skin, which subsequently appears as dermatitis, may lead to the suspicion that an allergy to the drug aggravates the dermatitis.

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Thus, patch tests are the only method of testing whether a certain substance aggravates dermatitis.

Preparation for the patch test

The patient should have a conversation with the specialist in which he should tell in as much detail as possible the origin of the skin problem affecting him, as well as the place where the rash started. He/she should also detail those treatments he/she has tried, as well as the family history and the products he/she encounters in his/her working life.

How do you feel during the test?

In the case of patch tests, we are talking about patches that are placed on the patient’s back for at least 24 hours. As such, the test is painless and causes little discomfort, apart from the discomfort of wearing a patch on the back and the impediment of washing the area and activities involving sweating.

Significance of abnormal results

Any reaction obtained will be noted by the specialist as follows:

– Negative reaction

– Doubtful reaction (moderate redness)

– Weakly positive reaction (reddened skin)

– Weakly positive reaction

– Strongly positive reaction

– Irritant reaction

However, there are a number of contraindications that may show unexpected or abnormal results, such as if the patient is taking systemic or topical corticosteroids, as well as avoiding exposure to the sun and avoiding performing the test on hot days, as the results may be interfered with.