Two Key Guidelines for the Assessment of Skin Irritation: OECD TG 404 and ISO 10993-23:2021
In the biocompatibility assessment of chemicals and medical devices, determining the local toxic effects that may result from skin contact is of critical importance. As the body’s largest organ, the skin is in constant interaction with the external environment and serves as the first line of defense against potentially harmful substances. Therefore, evaluating a product’s potential to cause skin irritation using accurate and reliable methods is essential for both user safety and regulatory compliance. Our laboratory applies the two fundamental international standards in this field—OECD TG 404 for chemicals and ISO 10993-23:2021 for medical devices—in accordance with GLP and GMP principles, using the most up-to-date scientific approaches.
OECD TG 404: Acute Dermal Irritation/Corrosion Test
OECD TG 404 is the classic and most widely accepted standard protocol used to evaluate the acute irritant and corrosive effects that chemical substances and mixtures can cause on intact skin. This test measures local reactions—specifically erythema (redness) and edema (swelling)—observed following brief (typically 4-hour) contact of a chemical with the skin, and grades their severity. The test substance is applied to a shaved, intact skin area of the test subject, covered with a semi-occlusive dressing, and removed after a specified contact time. Observations are made at 1, 24, 48, and 72 hours after removal of the substance, and erythema and edema scores are rated on a scale from 0 (none) to 4 (severe) at each observation point. The average of these scores is used to calculate the Primary Irritation Index (PII), and the substance is classified as “Skin Irritant” (Category 2) or “Skin Corrosive” (Category 1) according to GHS criteria.
In applying OECD TG 404, our laboratory fully complies with the 3R principles. In particular, if there is prior information regarding the test substance’s corrosive potential, or if its pH value is ≤2 or ≥11.5, in vivo testing, validated in vitro alternative methods (e.g., reconstituted human epidermis models under OECD TG 430, 431, or 435) must be applied first. This stepwise strategy ensures scientific validity while preventing unnecessary use of animals.
ISO 10993-23:2021: Skin Irritation Test for Medical Devices
ISO 10993-23:2021 is an international standard specifically developed to determine the potential irritant effects of medical devices that come into contact with the skin, as part of the biocompatibility assessment of medical devices. This guideline is one of the biological evaluation standards in the ISO 10993 series and represents an updated version of the irritation tests previously included in ISO 10993-10, now established as a separate standard. ISO 10993-23 evaluates the effects on the skin of extracts obtained from the device or the device material itself, in a manner that best reflects the medical device’s clinical usage scenario.
The most significant innovation of this standard is its emphasis on in vitro methods. ISO 10993-23:2021 recommends the use of validated in vitro reconstituted human epidermis (RhE) models as the first choice for skin irritation assessment. These models are three-dimensional tissue cultures that mimic the structural and functional characteristics of human skin. The test is based on the direct application of the device extract or material to this tissue model and the measurement of cell viability using the MTT assay. If cell viability falls below a specific threshold value (50%), the material is classified as “irritating.” This approach completely eliminates the use of animals while allowing for a more direct extrapolation to human skin. In vitro In cases where this method is not appropriate or the results are inconclusive, in vivo skin irritation testing (patch testing on rabbits or human volunteers) may be used as an alternative.
The table below provides a comparative overview of the key features of the OECD TG 404 and ISO 10993-23:2021 guidelines:
| Feature | OECD TG 404 | ISO 10993-23:2021 |
|---|---|---|
| Scope | Chemical substances, mixtures, pesticides, industrial products | Medical devices and biomaterials (wound dressings, catheters, surgical instruments, implants, etc.) |
| Test Type | Acute dermal irritation and corrosion | Skin irritation (biocompatibility assessment) |
| Primary Method | In vivo (rabbit model), preceded by in vitro screening using a stepwise strategy | In vitro (RhE model) preferred; in vivo as an alternative when necessary |
| Test System | Albino rabbit (typically New Zealand White), 3 subjects | Reconstituted human epidermis (EpiSkin™, EpiDerm™, SkinEthic™, etc.) or human volunteer patch test |
| Contact Time | 4 hours (semi-occlusive dressing) | Depending on the device’s clinical usage time (between 15 minutes and 24 hours) |
| Evaluation Parameters | Erythema score (0–4), Edema score (0–4), Primary Irritation Index (PII) | Cell viability (MTT assay, % viability), morphological changes, pro-inflammatory cytokine release (optional) |
| Classification / Output | GHS: Skin Corrosive (Cat. 1), Skin Irritant (Cat. 2), Mild Irritant (Cat. 3, optional), Not Classified | Irritant / Non-irritant; integrated into the device’s biocompatibility profile |
| 3R Compliance | Reduction and Refinement through a tiered strategy; in vitro alternatives (TG 430, 431, 435) are a prerequisite for substitution | High: In vitro methods are presented as the primary preference, largely eliminating the use of animals |
| Regulatory References | EU REACH, CLP, EPA FIFRA, GHS, KKDIK (Turkey) | EU Medical Device Regulation (MDR 2017/745), ISO 10993-1, FDA 510(k), MDR, IVDR |
| Extraction / Sample Preparation | Not applicable (test substance is applied directly) | Extraction with polar and non-polar solvents; temperature and duration conditions reflecting the device’s clinical use |
Innovations and Benefits Introduced by ISO 10993-23:2021
ISO 10993-23:2021 introduces significant innovations compared to its predecessor, ISO 10993-10. These innovations are critical updates that medical device manufacturers must consider in their regulatory submissions:
| Change | Description | Impact on Manufacturers |
|---|---|---|
| In vitro Priority | Validated RhE models have been officially recognized as the first step in skin irritation assessment. | Reliance on animal testing decreases; ethics committee approval processes are simplified; testing time is reduced. |
| Standardization of Extraction Conditions | Extraction temperatures and durations that reflect the device’s clinical use scenario have been clarified (e.g., 24–72 hours at 37°C for surface devices). | The alignment of test results with clinical reality improves; the risk of false-positive or false-negative results decreases. |
| Expanded Evaluation Criteria | The evaluation now includes not only cell viability but also morphological changes and inflammatory markers. | A more precise and comprehensive biocompatibility profile is obtained; clearer decisions can be made in borderline cases. |
| Testing the Device Itself | In addition to extracts, the option to apply the device material directly to the RhE model has been added. | This provides a more realistic exposure simulation, particularly for solid devices and surface coatings. |
| Integration with Risk Assessment | It explains how test results should be integrated into the overall risk management process under ISO 14971. | Biocompatibility data is placed within a more meaningful context within the device’s overall safety profile. |
Laboratory Selection Guide: Which Test for Which Product?
The table below will help you select the right test based on your product type and regulatory objective:
| Product Type | Recommended Guideline | Rationale |
|---|---|---|
| Industrial chemicals, pesticides, cleaning products, cosmetic raw materials | OECD TG 404 | It is an internationally recognized mandatory standard for GHS/CLP classification and SDS preparation. |
| Wound dressings, bandages, transdermal patches | ISO 10993-23:2021 | A requirement under the MDR for the biocompatibility assessment of medical devices intended for prolonged contact with the skin. |
| Catheters, surgical instruments, implants, prostheses | ISO 10993-23:2021 | An irritation test in accordance with ISO 10993-1 is required for all medical devices that come into contact with the skin and mucous membranes. |
| Finished cosmetic product (within the scope of the EU) | EU Cosmetics Regulation + OECD TG 439 (in vitro) | Animal testing for cosmetics is prohibited in the EU; validated in vitro alternatives are mandatory. OECD TG 404 is not suitable for these products. |
| Pharmaceutical active ingredient (topical formulation) | OECD TG 404 or ICH S10 | In drug development, phototoxicity and irritation are evaluated within the framework of ICH guidelines; OECD TG 404 may serve as a supplement. |
| Medical device coatings, adhesives, sterilization residues | ISO 10993-23:2021 | An evaluation within the scope of biocompatibility is required for all device components that come into contact with the patient. |
Conducting Skin Irritation Tests in Our Laboratory
In our GLP- and GMP-certified laboratory, we conduct skin irritation tests in accordance with international standards, including OECD TG 404 for chemical substances and ISO 10993-23:2021 for medical devices. For in vitro skin irritation tests, we use validated reconstituted human epidermis models (EpiSkin™, EpiDerm™, SkinEthic™); in cases where in vivo testing is unavoidable, we operate in full compliance with the 3R principles under the supervision of experienced veterinarians and toxicologists. Under both guidelines, we tailor your testing strategy to the specific requirements of your product and provide comprehensive, defensible reports for your regulatory submissions.
Frequently Asked Questions
| Question | Answer |
|---|---|
| Are the results from OECD TG 404 and ISO 10993-23 interchangeable? | Generally, no. OECD TG 404 is designed for the GHS classification of chemicals; ISO 10993-23, on the other hand, is specific to the biocompatibility profile of medical devices. It is mandatory to use the correct standard in regulatory submissions. |
| Do I need to have both OECD TG 404 and ISO 10993-23 testing performed for my medical device? | No. ISO 10993-23 alone is sufficient for medical devices and is accepted by international regulatory authorities. OECD TG 404 is intended for chemical substances and is not required for medical device submissions. |
| Is in vitro testing under ISO 10993-23 always sufficient? | In the vast majority of cases, yes. However, if the test results are inconclusive, if the device’s specific usage scenario cannot be modeled as in vitro, or if the regulatory authority specifically requests it, in vivo testing may be required. |
| Is it mandatory to conduct an OECD TG 404 test for a substance with an extreme pH value? | No. Substances with a pH ≤2 or ≥11.5 can be classified directly as “Skin Corrosive” without the need for an in vivo test. This approach is recommended by the OECD to prevent unnecessary use of animals. |
| How is a medical device extract prepared? | In accordance with ISO 10993-12, extraction is performed using specific volumes of polar (serum-containing medium) and non-polar (vegetable oil) solvents per unit surface area or mass of the device, at temperatures and for durations that reflect clinical use. |
| How long does it take to get skin irritation test results? | OECD TG 404 (in vivo): 2–3 weeks. ISO 10993-23 (in vitro RhE): 1–2 weeks. In vivo ISO 10993-23: 3–4 weeks. The total duration, including reporting and quality assurance review, is 3–6 weeks. |
The skin is the organ responsible for the first contact with your product, and scientifically proving that this contact is safe is both a legal requirement and the foundation of your brand’s credibility. Whether you need a skin irritation assessment under OECD TG 404 for a chemical substance or under ISO 10993-23:2021 for your medical device, our GLP-certified laboratory is here to support you with the latest methods and a team of experts. To demonstrate your product’s skin safety, contact us.