Patch Test: What It Is, How It Works and How to Do It Yourself

Patch tests are an important tool in dermatology for detecting skin reactions and allergies. But they are also used in the development of cosmetics – and you can even do a simple version yourself at home before using a new product. Here you'll find out how a patch test works, how to read the results and what matters when it comes to natural cosmetics.

The three types of patch test

The epicutaneous test at the dermatologist

The epicutaneous test is the standard test for confirming a contact allergy – for example if you keep getting eczema and a substance from jewellery, cosmetics or your work environment is suspected to be behind it.

Preparation

• The skin on your back should be healthy and not inflamed at the time of the test.
• Don't use any cortisone cream on your back for about a week beforehand, and don't expose your back to strong sun or a sunbed for a few weeks beforehand.
• If you take medication that affects the immune system (such as cortisone tablets), discuss this with your dermatologist beforehand.

Procedure and test duration

1. Application: The test substances are filled into small test chambers at defined concentrations and attached with plasters to the cleansed skin on your back. Usually a standard series with the most common contact allergens is tested, supplemented with further substances or your own products if needed.
2. Exposure: The plasters usually stay on the skin for 48 hours. During this time you shouldn't get your back wet, sweat heavily or do any sport.
3. Reading: After the plasters are removed, the skin is given about 30 minutes to recover from the pressure of the plasters before the first reading. Further readings usually follow after 72 hours and often once more after about a week, because allergic reactions frequently only appear with a delay.

Documentation

So that the results can be evaluated reliably, everything is recorded systematically:

• date and time of application,
• name and concentration of each test substance,
• the exact position of each plaster,
• type and strength of the skin reactions at each reading.

Standardised documentation forms and control fields without a test substance ensure that the results remain comparable.

Reading the test results

The results are assessed using an internationally uniform scale that goes back to the ICDRG (International Contact Dermatitis Research Group):

Irritant or allergic?

There are two main types of skin reaction, which can be told apart by their characteristics:

• Irritant reactions (irritation) tend to show up as sharply defined redness, scaling or shiny skin that looks "as if burnt". They are often strongest right after removal and then fade.
• Allergic reactions show up as redness with swelling, small bumps (papules) or blisters (vesicles). They often spread beyond the test area and tend to get stronger in the days after removal.

At first, a positive reaction only means that your skin is sensitised to the substance. Whether it is really responsible for your symptoms is assessed by your dermatologist together with your medical history. You will then usually receive an allergy passport.

Standards and guidelines

To make patch tests reliable and comparable, they follow fixed rules:

• For the epicutaneous test at the dermatologist, there are guidelines from the professional societies – in Germany, for example, from the German Contact Allergy Group (DKG), and internationally from the European Society of Contact Dermatitis. They define which substances are tested at which concentration, how long the plasters stay on and when the readings take place.
• For cosmetics, the EU Cosmetics Regulation requires every product to undergo a safety assessment before it is sold. Animal testing for cosmetics is banned in the EU. Tolerance tests on volunteers, as carried out by testing institutes, complement this assessment.
• The standard DIN EN ISO 10993-10 is sometimes mentioned in connection with patch tests. However, it governs the testing of medical devices for skin sensitisation – not of cosmetics.

For reliable results, a few basic rules also apply:

• Test materials: verifiable purity, stability throughout the entire test period and defined concentrations.
• Consistency: standardised reading times, control fields and trained assessors who evaluate uniformly.

How natural cosmetics are tested for tolerance

Plant-based ingredients can also trigger allergies – "natural" doesn't automatically mean "well tolerated". Plant extracts contain many different substances, and some plants, such as those from the daisy family like chamomile or arnica, are known allergy triggers. That's why checking tolerance is especially important for natural cosmetics. The safety assessment clarifies what role the concentration of an extract plays in this.

This is how a tolerance test at Dermatest works, as we have had it carried out for our products:

1. 30 test subjects with sensitive skin take part.
2. The product is applied to the skin at a suitable concentration under a test plaster.
3. After 24, 48 and 72 hours, the skin is examined for irritation such as redness, each time 30 to 60 minutes after the plaster has been removed.
4. If more than one in ten test subjects shows skin irritation, the test is failed.

With our tested products, none of the test subjects experienced skin irritation – they were rated "SEHR GUT" (very good). You can read more about this in Our psoriasis products: clinically tested with Dermatest.

natüür safety standards

Our active ingredient complex NEEMAVERA® is obtained from pure neem leaves by CO₂ extraction and concentrated many times over in the process. Our Med Care products, including those for sensitive and irritated skin such as with psoriasis or atopic dermatitis, are:

• dermatologically tested for tolerance (Dermatest "SEHR GUT"),
• free from parabens, silicones, artificial colourings and fragrances,
• vegan and made in Germany.

We explain which natural ingredients are good for sensitive skin in Natural ingredients for sensitive skin. And you can read there what a gentle skincare routine for psoriasis or atopic dermatitis can look like.

Patch test at home: how to test new cosmetics

Especially if you have sensitive skin, a quick self-test is worth it before you use a new product over a large area:

1. Choose a spot: Apply a small amount to the inside of your forearm, the crook of your elbow or behind your ear – for facial care, a small spot along your jawline works well too.
2. Repeat: Apply the product to the same spot once or twice a day for two to three days, just as you would use it later.
3. Observe: Look out for redness, itching, burning, swelling or blisters – including for one to two days after the last application.
4. Evaluate: If your skin stays calm, you can carefully start using the product normally. If it reacts, wash the area and stop using the product.

Important: A self-test is no substitute for an allergy test at the dermatologist. With hair dyes, a test 48 hours before every use is particularly important – manufacturers provide instructions for this on the packaging. If you react strongly to a product or keep getting eczema, have an epicutaneous test done.

Summary

Standardised patch tests are crucial for detecting contact allergies and developing safe skincare. They help to distinguish between irritant and allergic reactions, to determine suitable concentrations and to check suitability for sensitive skin. Testing tolerance is particularly important for natural cosmetics, because plant substances can also cause irritation. And with a simple self-test at home, you can find out whether a new product suits your skin.

Frequently asked questions

What is a patch test?

A patch test (epicutaneous test) is a skin test in which test substances are applied to the skin under plasters to identify contact allergies or skin irritation.

How long does a patch test at the dermatologist take?

The plasters usually stay on your back for 48 hours. Readings are taken after removal, after 72 hours and often once more after about a week. So you'll need several appointments in total.

Does a patch test hurt?

No. The plasters are simply stuck on – there are no needles involved. If there is a positive reaction, however, the test site may itch.

How do I do a patch test at home?

Apply the new product once or twice a day for two to three days to a small spot on the inside of your forearm, in the crook of your elbow or behind your ear, and watch for redness, itching or burning.

Does health insurance pay for the epicutaneous test?

If a doctor suspects a contact allergy, German statutory health insurance funds usually cover the costs. If in doubt, ask your practice beforehand.