What is the official description of skin irritation?

Skin irritation refers to the production of reversible damage to the skin following the application of a test chemical for up to 4 hours [as defined by the United Nations (UN) Globally Harmonized System of Classification and Labelling of Chemicals (GHS)] (from OECD TG 439)

What is the current OECD Guideline for skin irritation testing?

The current OECD Test Guideline / OECD Ligne Directrice 439 "OECD GUIDELINES FOR THE TESTING OF CHEMICALS; In Vitro Skin Irritation: Reconstructed Human Epidermis Test Method" (OECD TG 439).

Here, we provide an extract of this Test Guideline. If you need more detailed information, please consult the full guideline as PDF.

[...] This Test Guideline (TG) provides an in vitro procedure that may be used for the hazard identification of irritant chemicals (substances and mixtures) in accordance with UN GHS Category 2. In member countries or regions that do not adopt the optional UN GHS Category 3 (mild irritants), this Test Guideline can also be used to identify non-classified chemicals. Therefore, depending on the regulatory framework and the classification system in use, this Test Guideline may be used to determine the skin irritancy of chemicals either as a stand-alone replacement test for in vivo skin irritation testing or as a partial replacement test within a tiered testing strategy. [...]

[...] This Test Guideline addresses the human health endpoint skin irritation. It is based on the in vitro test system of reconstructed human epidermis (RhE), which closely mimics the biochemical and physiological properties of the epidermis. The RhE test system uses human derived non-transformed keratinocytes as cell source to reconstruct an epidermal model with representative histology and cyto-architecture. [...]

[...] Pre-validation, optimization and validation studies have been completed for four commercially available in vitro test methods based on the RhE test system. These four test methods are included in this TG and are listed in Annex 2, which also provides information on the type of validation study used to validate the respective test methods. [...]

[...] While this Test Guideline does not provide adequate information on skin corrosion, it should be noted that OECD TG 431 on skin corrosion is based on the same RhE test system, though using another protocol. [...]

[...] This Test Guideline is based on RhE-models using human keratinocytes, which represent in vitro the target organ of the species of interest. Moreover it directly covers the initial step of the inflammatory cascade/mechanism of action (cell and tissue damage resulting in localized trauma) that occurs during irritation in vivo. [...]

[...] The Test Guideline is applicable to solids, liquids, semi-solids and waxes. [...]

How does the test work?

The test chemical is applied topically to a three-dimensional RhE model, comprised of non-transformed human-derived epidermal keratinocytes, which have been cultured to form a multilayered, highly differentiated model of the human epidermis...
... the RhE-based test methods (...) measure the initiating events in the cascade, e.g. cell / tissue damage, using cell viability as readout.

Cell viability in RhE models is measured by enzymatic conversion of the vital dye (...) into a blue formazan salt that is quantitatively measured after extraction from tissues. Irritant chemicals are identified by their ability to decrease cell viability below defined threshold levels (i.e. ≤ 50%, for UN GHS Category 2). Depending on the regulatory framework and applicability of the Test Guideline, chemicals that produce cell viabilities above the defined threshold level, may be considered non-irritants (i.e. > 50%, No Category).

According to the test procedure, it is essential that the viability measurement is not performed immediately after exposure to the test chemical, but after a sufficiently long post-treatment incubation period of the rinsed tissue in fresh medium. This period allows both for recovery from weak cytotoxic effects and for appearance of clear cytotoxic effects. A 42 hours post-treatment incubation period was found optimal during test optimization of two of the RhE-based test methods underlying this TG.


Fig. 1 Illustration of the testing protocol

How can I translate viability data into substance classification?

Viability measured after 20 min exposure
and 42 h post incubation 
Prediction to be considered 
≤ 50% Irritant

Category 2 (Cat. 2) 
> 50% Non - irritant

Non - classified

How does this work in practice?

This case study illustrates skin irritation testing results and the classification of test chemicals for two test substances as an example.

Case study skin irritation

Case A:

Substance A is a solid substance, which is applied as a powder (after grinding) topically onto the epidermis.

Exposure time 20 min
Viability 73 %
Classification No Category (non irritant)

Case B:

Substance B is a creamy substance, which is applied with a brush topically onto the epidermis.

Exposure time 20 min
Viability 39 %
Classification Category 2; irritant

Which methods are validated and accepted?

Four methods are validated: EpiSkinTM, EpidermTM, SkinEthicTM RHE and LabCyteTM.

The epiCS® method for skin irritation testing is completed and reveals very good predictivity and reliability:

epiCS methodOther validated test methods
Sensitivity100 %70 - 90 %
Specificity80 %73,3 - 80 %
Overall Accuracy90 %75 - 85 %

The epiCS method showed a "Within Laboratory Reproducibility" of 96,58 % and "Between Laboratory Reproducibility" of 95 %.

Data of the epiCS Skin Irritation Testing (SIT) study were handed in to EURL-ECVAM for reviewing and for submitting to the Scientific Committee of ECVAM for peer review. The respective ESAC statement is expected end 2015.
The epiCS SIT method will also be integrated in the OECD/OCDE TG 439.

What do I need for skin irritation testing?

We have developed a specific kit that you can use for in vitro skin irritation testing. In addition you need laboratory material which are standards in cell culture laboratories.

Kit content:

  • epiCS (tissues)
  • epiCS Culture Medium - to culture the tissues - for three medium changes (sufficient for skin irritation testing)
  • epiCS MTT-Assay Medium - for MTT-Assay
  • 6-well plates
  • Lot specific epiCS certificate of analysis

For further Information, please check the epiCS SOP and manual: 

SOP epiCS in vitro Skin Irritation, v4.0, 2012

epiCS Manual, 2014

We provide competent swift technical and scientific support by telephone or e-mail.

How can I compare test performances in different labs?

We have designed a validation kit for skin irritation to conduct a multicenter study to validate between laboratory reliability.

The concept:

Two (blinded) test substances are tested at your site(s) and at our ISO9001:2008 certified laboratories. Testing is carried out at the same time on the same lot of tissues to minimize material variations between the sites. Test results are compared and in case of concordant results a certificate is issued by CellSystems to the qualified user. This study can be carried out with one or more external laboratories at the same time.

Who should participate?

a) Beginners - to prove that the test method is carried out correctly following the SOP.
b) Routine users - to prove on a routine basis the reproducible performance of the technicians.

For further information, please download the PDF below

SOP epiCS in vitro Skin Irritation, v4.0