Podcast: Episode 13, patch tests and HRIPT

Audio Episode, Test Patches and HRIPT


The Patch Test is a clinical method used in allergology and cosmetology to assess the irritant and sensitizing potential of a cosmetic. There are several protocols:

The 48-hour test patch

  • it's the cheapest and fastest
  • assess the irritant potential of a cosmetic
  • generally and preferably out of 50 volunteers
  • consists of applying the cosmetic to the volunteers, covering it with a bandage (occlusive application) and keeping it on for 48 hours. After 48 hours, the dressing is removed and the skin is assessed by an expert/dermatologist. The results of the panel are analyzed and a report concluding the irritant potential of the cosmetic is written.


  • it is the most expensive and the slowest
  • allows you to assess the sensitizing potential of a cosmetic (+ irritant potential)
  • generally and preferably out of 50 volunteers
  • Consists of 3 test phases: the sensitization phase, the rest phase and the elicitation phase.
  • awareness phase: consists of applying the cosmetic to the volunteers, covering it with a bandage (occlusive application) and keeping it on for 24 hours. When the dressing is removed, the skin is assessed by an expert/dermatologist and the cosmetic is reapplied to the same application area to be kept on again for 24 hours. In all the cosmetic is applied 9 times on the same area.
  • rest phase: after the sensitization phase, the volunteer does not have a test for 2 weeks to allow their body to potentially manufacture the proteins necessary to cause an allergic reaction.
  • elicitation phase: after the rest phase, the cosmetic is reapplied to a different area of ​​the volunteer. The skin is evaluated by an expert/dermatologist.
  • after the 3 test phases, the results of the volunteers are evaluated for the writing of the concluding report on the sensitizing potential of the cosmetic

Episode Notes


Hello, you are listening to episode 13 of Les Sous de la Cosmétique, the Podcast which reveals and explains to you with clarity and sincerity the world of cosmetics. I am Julie Magand Castel, chemical biologist and cosmetologist specialized in natural cosmetics and today we are going to talk about patch tests. And yes, we are going to talk about their usefulness, their test protocols, their regulatory aspect, their differences because there are different patch test protocols and also more practically their cost.

So the patch test is a general term to define a method used in allergology to determine if a person is allergic to a substance. But there are different protocols, and depending on the protocol it can also be used to determine if a substance is irritating. It consists of applying the substance to be analyzed to a person or a panel and then covering or not covering the substance in an occlusive or semi-occlusive manner, using a kind of dressing. Then the person stays with the bandage for one or two days. The dressing is removed, the skin is assessed immediately and / or after one or more days.

In allergology, the patient explains his allergic situation, he discusses with the doctor and together determines the suspicious substances in his daily life. And then the doctor applies these substances generally in the back and well organized in the form of a grid a small edge of the back is devoted to a substance. The substance is covered with a dressing for occlusive application. Because the occlusive application increases the penetration of the substance into the skin and maximizes the risk of reaction. The dressing is kept for two days and the skin is assessed when it is removed, is it red? is it swollen? And it can be evaluated again 1 or 2 days after removal.

This was the case where a patient suffers from an allergy and we are trying to determine which substance his immune system reacts to, so a patient and several substances. In cosmetics, we don't treat patients, we test formulas. So it's the opposite, we test a formula on several volunteers called the panel. There are different operating modes for the test patches, more or less long, more or less expensive, more or less strict.

The cheapest and fastest is the 48h patch test. Generally it is done on 50 volunteers and it is used to determine if the cosmetic is irritating. Then there is the HRIPT which is longer and more expensive and it is used to determine if the cosmetic is sensitizing / if it is allergenic, it is also generally done on 50 volunteers.

So I would like to take the time to differentiate between an irritation and an allergy. An irritation is an inflammatory reaction that occurs on the surface of the skin. It can result in redness and dryness at the site of exposure. It will touch the epidermis cells for a while and it will go away. It is a temporary reaction which is caused by the substance applied and which will occur in most individuals.

On the contrary, an allergic reaction is a reaction caused by our immune system in response to contact with a specific substance. An allergic reaction in one individual will not predict a potential allergy in a second individual. I have a girlfriend who is allergic to strawberries, but I don't have an allergic reaction to strawberries. Whereas if we both put bleach on our hands, we're both going to get an irritation.

I am qualifying my suggestions a little because there are many substances, particularly in perfumery, which are known to be allergenic for a greater number of people. An allergic reaction may result in itching, rashes and is not limited to the site of exposure.

Now that we have made the distinction between sensitization and irritation, we can see in a little more detail the differences between a 48h patch test and a RIPT.

The 48-hour patch test is generally done on 50 people, we put 0.2 mL of product on 2cm2 of skin, generally on the back or sometimes hard there inside the forearms and we cover with a bandage. We leave the bandage in place for 48 hours, of course you should not remove the bandage during this time and do not play sports to avoid perspiration. Then the dressing is removed and the appearance of the skin is assessed by an expert, usually a dermatologist. It then evaluates the irritant character.

Sometimes, but it's rare, the skin is also reassessed 1 or 2 days later, but in all cases volunteers are asked to report any problems after the patch test. And then it's the last phase where we do analyzes on the results and we rewrite the report with the conclusion - that is to say, is the product considered to be irritating. It is a quick test and does not cost the brand very expensive, around 200 euros.

The HRIPT is widely recognized in the industry, it is much more demanding and more expensive because it costs around 1300 euros. HRIPT stands for Human Repeated Insult Patch Test. It is broken down into 3 phases. The first phase is sensitization, the panel of generally 50 volunteers and exposed 9 times to the substance on the same contact area. They keep the substance covered with the dressing for 24 hours, he removes the dressing and the area is assessed 48 hours after the dressing is removed.

The protocol of some labs does not include these 48hrs and therefore the skin is evaluated directly after removal and a new dressing is reapplied. Then we repeat this 8 times. Then there is a 2 week rest period which allows the body to potentially manufacture the proteins which would cause an allergic reaction to the next contact. Then the last phase is the elicitation phase, we reapply the product to another part of the body and we evaluate to see if there has indeed been a sensitization phase and if the immune system reacts. The first phase itself can already be used to assess whether the product is irritating. And then the third phase assesses whether the product is sensitizing.

I already see you wondering why in allergology we evaluate the allergic reaction with a protocol that ultimately resembles that of a 48-hour patch test that evaluates irritation. In fact in allergology, the patient has already been sensitized to the allergen so it is not necessary to do the first sensitization phase that we do in HRIPT, in the end it is as if we were only doing the licitation phase in allergology. Then, in allergology, we only present non-irritating substances, we are not going to put on the patient who is already allergic a substance known to be irritating.

These tests are good evidence to include in the DIP, the product information file to prove the safety of the cosmetic. The DIP is the file to be compiled for the marketing of a cosmetic in Europe. It can be quite long with around 150 pages.

To conclude, it is necessary for the brand to assess the safety of its product before placing it on the market. This notably involves a series of patch tests carried out on a panel of volunteers. The most widely used patch test protocols are the 48-hour patch test which evaluates the irritant potential of a product and the HRIPT which evaluates the sensitizing potential of a product - and also a little the irritant potential.

I hope you enjoyed this episode and convinced you of the need for this type of test. A new episode will be released next Saturday and you can find the notes for these episodes on mastelcosmetics in the under the cosmetics section. Mastel is written MASTEL and it is the contraction of my two surnames Magand Castel.

You can also subscribe to the Podcast Les sous de la beauté on various Podcast platforms including Spotify, Apple Podcast and Google Podcast and follow its news on Instagram and Twitter with the username mastel_cosmetics. Also and of course feel free to share this podcast with your friends if you think they might enjoy it - thank you. I wish you a great week and look forward to seeing you next Saturday for an episode on skin aging, including, among other things, how this translates into the biological functions of our skin.


Leave a comment

Please note, comments must be approved before they are published

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.


A chemist in the cosmetics industry for more than 5 years and a graduate of the Natural Raw Materials in Cosmetics Master's degree from ISIPCA, Julie is an expert in the development of natural cosmetic products.