USP <381> Explained: Elastomeric Closures for Injections
USP <382>, the chapter was retitled and its scope refocused. This guide explains what USP <381> covers now, how it relates to USP <382>, where biological reactivity testing fits, and how the stopper works alongside the aluminium seal in the finished package.
Key takeaways
- USP <381> is the standard for elastomeric components (rubber stoppers) used with injectables. It is published in the United States Pharmacopeia-National Formulary (USP-NF).
- It covers physico-chemical tests (which typically include appearance of the extract solution, turbidity, acidity or alkalinity, absorbance, reducing substances, and extractable elements) plus biological reactivity testing of the material.
- It works in tandem with USP <382> Elastomeric Component Functional Suitability, which covers functional-suitability tests such as penetrability (penetration force) and self-sealing capacity. The fragmentation test is retained in <381>.
- Biological reactivity is assessed under USP <87> (in vitro) and USP <88> (in vivo), referenced from <381>.
- The stopper is only one half of the seal: the aluminium or aluminium-plastic seal crimps over it to hold it in place and maintain container closure integrity.
What is USP <381>?
USP <381> is the United States Pharmacopeia general chapter titled “Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems,” and it specifies the test framework for the elastomeric (rubber) materials used to close injectable drug containers. It is published in the USP-NF, the official compendium of the United States Pharmacopeial Convention, and applies to products marketed in the United States.
An elastomeric closure is the rubber stopper (or rubber-faced component) that seals a vial, bottle, or cartridge holding an injectable medicine. Because the stopper is in direct contact with the drug and is pierced by a needle, the material must be chemically clean enough not to leach harmful substances into the product, and biologically safe enough not to provoke an adverse reaction. USP <381> defines how that material is identified and characterized.
USP <381> is a material chapter: it characterizes the rubber itself. It is read together with USP <382>, which characterizes how the finished closure performs in use. Both chapters are maintained by the United States Pharmacopeial Convention and revised through its standards-setting process.
What USP <381> tests
USP <381> sets out physico-chemical tests on extracts of the elastomeric material plus biological reactivity tests, so that a closure can be shown to be chemically suitable and biologically safe for injectable use. The chapter groups its requirements into two broad areas.
Physico-chemical tests
These tests examine an aqueous (and where specified, other) extract of the closure to confirm the material does not release problematic substances. Physico-chemical attributes evaluated for elastomeric closures typically include the appearance and turbidity of the extract solution, acidity or alkalinity, UV absorbance, reducing (oxidizable) substances, and extractable elements. The intent is to demonstrate the rubber is consistent and chemically clean, which is foundational evidence ahead of any product-specific extractables and leachables program. The official chapter is the authority on which attributes apply and how they are measured, so confirm the current list against the USP-NF.
Biological reactivity
USP <381> requires the closure material to meet biological reactivity expectations, assessed using the general chapters for biological reactivity in vitro (USP <87>) and in vivo (USP <88>). These tests evaluate whether the material, or substances it releases, cause a cytotoxic or other biological response, confirming the elastomer is appropriate for contact with a parenteral product.
Because the precise acceptance criteria and procedures are version-controlled in the compendium, manufacturers and buyers should always work from the current official text in the USP-NF rather than from a summary.
How USP <381> relates to USP <382>
USP <381> covers what the elastomeric material is (physico-chemical and biological characterization), while USP <382> covers how the finished closure performs as part of a parenteral packaging and delivery system. They are complementary chapters, and an injectable closure is generally evaluated against both. In the USP-NF restructure, penetrability and self-sealing capacity were placed in <382>, while the fragmentation test was retained in <381>.
| Aspect | USP <381> | USP <382> |
|---|---|---|
| Focus | The elastomeric material | The finished closure in the delivery system |
| Type of testing | Physico-chemical and biological reactivity (and fragmentation) | Functional suitability |
| Example attributes | Extract appearance, absorbance, reducing substances, biological reactivity | Penetrability and self-sealing capacity |
| Question it answers | Is the rubber chemically and biologically suitable? | Does the closure perform with the container and the drug? |
In practice, <381> establishes that the rubber is a clean, safe material that does not fragment unacceptably when pierced, and <382> establishes that the stopper, in the specific container closure system, can be penetrated and reseals after needle withdrawal. Because the precise allocation of tests is version-controlled, always confirm against the official chapters. For a fuller picture of the functional side, see our guide to USP <382> functional suitability of elastomeric components.
Where biological reactivity testing fits
Biological reactivity for elastomeric closures is evaluated under the general chapters USP <87> (in vitro) and USP <88> (in vivo), which USP <381> references. USP <87> uses cell-culture based methods to detect a cytotoxic response from the material or its extracts. USP <88> uses in vivo methods and a classification system to assess the biological response to plastics and other polymers, including elastomers, and is the basis for the older “Class VI” terminology often quoted for pharmaceutical materials.
For a closure that contacts an injectable, demonstrating acceptable biological reactivity is part of showing the material is fit for parenteral contact. This sits alongside, and feeds into, the broader material-safety picture covered by extractables and leachables studies and toxicological assessment.
The stopper and the aluminium seal: one system
The elastomeric stopper governed by USP <381> and the aluminium seal that crimps over it together form the closure of the injectable container closure system. Neither component works alone. The rubber stopper provides the sterile barrier and the resealable piercing surface; the aluminium or aluminium-plastic seal is crimped around the vial neck and over the stopper flange to hold the stopper firmly in place and maintain compression on the sealing surface.
If the seal does not crimp the stopper correctly, the stopper can lose compression and the container closure integrity of the package is at risk, even if the rubber itself fully meets USP <381>. This is why the standards for the elastomeric closure and the way the primary packaging components are assembled are evaluated as part of one system rather than in isolation. The aluminium seal does not contact the drug the way the stopper does, but it is what keeps the qualified stopper performing as intended across the product’s shelf life.
How this works in practice at Autofits
Autofits manufactures the aluminium-plastic FlipTop® seals and aluminium pilfer-proof caps that crimp over the elastomeric stopper to complete the injectable container closure system. Autofits does not make the rubber stopper itself, but its seals, including the FlipTop Optima flip-off seals used on vaccine and parenteral vials, are engineered to crimp consistently over the stopper flange and hold the closure under controlled compression, which is what allows a USP <381> compliant stopper to do its job once the vial is sealed. Production runs under an ISO 15378:2017 quality system, alongside ISO 9001:2015 and ISO 14001:2015 certification and a Drug Master File (DMF), in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom and high-speed visual inspection on the closure lines. You can review the full set of certifications on the quality page.
Frequently asked questions
What is USP <381>?
USP <381> is the United States Pharmacopeia general chapter titled “Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems” (historically “Elastomeric Closures for Injections”). It defines the physico-chemical and biological reactivity test framework for the rubber closures (stoppers) used to seal injectable drug containers, and is published in the official USP-NF compendium.
What is the difference between USP <381> and USP <382>?
USP <381> characterizes the elastomeric material itself through physico-chemical and biological reactivity testing (and it retains the fragmentation test), answering whether the rubber is chemically and biologically suitable. USP <382> covers functional suitability, testing how the finished closure performs in use, including penetrability (penetration force) and self-sealing capacity. Injectable closures are generally evaluated against both chapters.
Does USP <381> cover rubber stoppers?
Yes. The elastomeric closures USP <381> addresses are the rubber stoppers (and rubber-faced components) that seal vials, bottles, and cartridges for injectable medicines. The chapter sets the material characterization framework for these components.
How does USP <381> relate to biological reactivity testing?
USP <381> references the general chapters USP <87> (in vitro biological reactivity) and USP <88> (in vivo biological reactivity) to confirm that the elastomeric material does not cause an unacceptable biological response. These chapters underpin the safety case for using the rubber in contact with a parenteral product.
Where does the aluminium seal fit relative to USP <381>?
The aluminium or aluminium-plastic seal is crimped over the USP <381> elastomeric stopper to hold it in place and maintain compression, completing the closure of the container closure system. The seal does not contact the drug the way the stopper does, but it is what keeps a qualified stopper sealing correctly over the product’s shelf life.
Is USP <381> mandatory?
USP-NF standards are enforceable for drug products marketed in the United States, so elastomeric closures used with such products are expected to conform to the relevant compendial requirements. Buyers and manufacturers should always confirm requirements against the current official text in the USP-NF.
Related reading
- USP <382>: functional suitability of elastomeric components
- ISO 15378:2017 GMP quality for primary packaging
- What a container closure system is
- What “primary packaging” means
- Extractables and leachables in vial closures
Sources
- USP: United States Pharmacopeia-National Formulary (USP-NF) (https://www.uspnf.com/)
- USP: USP General Chapter <381> Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems, in the USP-NF (https://www.uspnf.com/)
- USP: USP General Chapter <382> Elastomeric Component Functional Suitability in Parenteral Product Packaging/Delivery Systems, in the USP-NF (https://www.uspnf.com/)
- USP: USP General Chapters <87> and <88> Biological Reactivity Tests, in the USP-NF (https://www.uspnf.com/)
*Last updated: 2026-06-10. This article is general regulatory information, not legal or compliance advice; confirm current chapter requirements with the official USP-NF.*
