Patch test

Dr. med. Philipp Nicol is a freelance writer for the medical editorial team.

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The patch test is a skin test that can be used to determine whether and which substances cause a contact allergy. To do this, the doctor applies various substances that could trigger allergies (allergens) to the patient's skin (epicutaneous = "on the skin"). Read more about the patch test, when it is used and what risks it entails.

What is a patch test?

The patch test is a skin test that is used to diagnose so-called contact allergies (allergic contact dermatitis) - such as a nickel allergy. They arise through prolonged, direct skin contact with the relevant triggering substance (e.g. nickel-containing necklace). Because the hypersensitivity reactions occur with a delay, doctors speak of allergic reactions of the late type (type IV).

Colloquially, the patch test is also called the plaster test. Here, the doctor sticks special plasters with the allergy triggers (allergens) to be tested on the patient. Within a few days it can be seen from the corresponding skin reactions whether there is actually a contact allergy to one of the test substances.

When do you perform a patch test?

The dermatologist uses a patch test if a contact allergy is suspected. This can be the case, for example, with recurring, unclear skin changes.

What do you do with a patch test?

Over 4000 allergens can be tested with a patch test. Standardized test solutions are available for this. The most common cause of allergic contact dermatitis is nickel.

For the test, the doctor puts special skin patches on the patient's back or forearm. The plasters contain potential allergy triggers in small aluminum chambers (usually diluted in petroleum jelly). They usually stay on the skin for a day or two. When you lose weight, your doctor will check whether you have hypersensitivity reactions on the skin in one or more of the areas in question. He will check this a second time one or two days after removing the patch - sometimes allergic reactions to a contact allergen take longer to develop.

What are the risks of a patch test?

For the test allergens, experts recommend concentrations that experience has shown to not trigger any inflammatory reactions and, as a rule, do not cause any new hypersensitivity. If so, one speaks of primary sensitization. So far, this has only rarely been observed with some strong allergens. However, because the risk cannot be ruled out in principle, the doctor should not carry out the patch test with substances with which the patient has not yet come into contact.

In patients with sensitive skin, the build-up of heat and moisture under the patch can irritate the affected areas of the skin.

In rare cases, the patch test can lead to anaphylactic shock, in the worst case with loss of consciousness and cardiovascular failure.

What do I have to consider during the patch test?

As long as you have put the patches on for the patch test, you should not shower, exercise or sweat profusely.

Basically, the patch test is a safe and reliable allergy test, with which the trigger of a contact allergy can usually be identified after a few days.

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