ISO 11140-1 Chemical Indicator Classes Explained

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ISO 11140-1 Chemical Indicator Classes Explained: Which One Does Your Sterilisation Department Need?

Chemical indicators used in medical device sterilisation are classified under ISO 11140-1 into six classes — Class 1 through Class 6 — each with a different function, placement, and level of process assurance. Selecting the correct class for each application is a regulatory and best practice requirement for sterile services departments, CSSD units, theatre sterilisation teams, and any facility that processes reusable medical devices. This guide explains what each class does, when to use it, and how they relate to common sterilisation systems including steam, hydrogen peroxide, ethylene oxide, and low temperature steam-formaldehyde.

Why Chemical Indicators Matter in Medical Device Sterilisation

Sterilisation of reusable medical devices is a critical patient safety process. A device that is not sterile when used on a patient presents a direct risk of infection and, in surgical or interventional procedures, potentially fatal consequences. Sterilisation departments operate under strict regulatory frameworks — including HTM 01-01 in England, SHTM 01-01 in Scotland, and the requirements of BS EN ISO 17665 (steam sterilisation) and BS EN ISO 11135 (ethylene oxide sterilisation) — and must demonstrate through documentation that every sterilisation cycle has been performed correctly.

Chemical indicators do not sterilise anything. What they do is provide visible evidence — a colour change — that specific physical or chemical conditions associated with effective sterilisation were present during the cycle. Used correctly and in combination with biological indicators and physical cycle records, they form part of the layered assurance system that sterile services departments rely on to release processed loads with confidence.

ISO 11140-1 is the international standard that defines the performance requirements, test methods, and labelling requirements for chemical indicators used in sterilisation. Understanding which class does what is the foundation of any correctly designed monitoring system.

The Six Classes Under ISO 11140-1

Class 1 — Process Indicators

Purpose: To distinguish between processed and unprocessed items at a glance.

Class 1 indicators — most commonly autoclave tape — are designed to be applied to the outside of instrument packs, trays, or wrapped sets before sterilisation. They change colour when exposed to the sterilisation process, providing a simple visual confirmation that the pack has been through a cycle and is not unprocessed stock sitting alongside processed items.

Class 1 indicators do not confirm that sterilisation conditions were met. They confirm only that the item was exposed to the sterilisation process. This distinction is important: a Class 1 indicator on the outside of a pack that was placed in an under-loaded or malfunctioning autoclave may still show a colour change, even if sterilisation was not achieved.

Typical products: Autoclave indicator tape (steam, EO, hydrogen peroxide variants). Available in rolls for application to wrapped packs and rigid containers.

Where to use: On the outside of every pack, tray, or container entering the steriliser. Also used on sterilisation bags and pouches to confirm the pack has been processed.

Class 2 — Indicators for Use in Specific Tests

Purpose: To detect specific sterilisation failures, not to measure process variables.

Class 2 encompasses indicators designed for specific defined test procedures rather than routine load monitoring. The most important example is the Bowie Dick test (also called the air removal test or Type B test), which is used to assess the performance of pre-vacuum steam sterilisers. The Bowie Dick test checks whether the steriliser is capable of adequately removing air from the chamber — a precondition for steam penetration into wrapped loads. It is performed at the start of each day before the first processed load.

A Class 2 indicator used in a Bowie Dick test contains a chemical indicator sheet that should show a uniform colour change across its full area if air removal is satisfactory. Non-uniform or incomplete colour change indicates inadequate air removal, typically caused by air leaks, vacuum pump failure, or steam quality issues. The steriliser must not be used for patient-critical loads until the fault has been investigated and resolved.

Class 2 indicators are steriliser-specific, not load-specific: the result tells you something about the steriliser's performance, not about any individual pack within a load.

Typical products: Bowie Dick test packs (single-use and reusable). Available in standard and small configurations for different steriliser chamber sizes.

Where to use: In a pre-vacuum steam steriliser at the start of each day, before the first processed load. Specific placement requirements vary by steriliser model — follow your steriliser manufacturer's instructions and the guidance in HTM 01-01 or your relevant national standard.

Class 3 — Single Variable Indicators

Class 3 indicators respond to a single critical variable of the sterilisation process — most commonly temperature alone. They are now rarely specified in contemporary sterile services practice, having been largely superseded by Class 4 and Class 5 indicators that respond to multiple critical variables. They are not included in the current SteriTec range and are not discussed further here.

Class 4 — Multi-Variable Indicators

Purpose: To confirm that multiple critical process variables were met inside the load.

Class 4 indicators are designed to be placed inside instrument packs, trays, or pouches — at the point of packaging, before sterilisation — where they respond to two or more critical variables of the sterilisation process (typically time and temperature, or temperature, time, and steam quality). After the cycle, the colour change on the indicator, when retrieved from within the pack, confirms that those critical conditions were present at that location inside the load.

This is a significant step up from Class 1. A Class 4 indicator inside a pack tells you something about what happened inside that specific pack, not just that the pack went through the steriliser. It is particularly valuable for detecting cold spots within a complex or heavily loaded cycle.

For steam sterilisation, Class 4 indicators are typically rated to specific cycle parameters: 121°C, 132°C, or 134°C. It is important to select the grade matched to the cycle your steriliser runs, not a lower-rated grade that would give a positive result under sub-optimal conditions.

Typical products: SteriTec Cross Checks L — for steam processes at 121°C, 132°C, and 134°C. Also SteriTec Dry Checks — for dry heat sterilisation processes.

Where to use: Inside instrument packs and trays, typically placed alongside the device most challenging to sterilise (e.g. the most complex lumened instrument). Also used in test packs for validation purposes.

Class 5 — Integrating Indicators

Purpose: To provide the highest level of chemical assurance by integrating across all critical sterilisation variables.

Class 5 indicators — commonly known as integrators — are the most demanding chemical indicator class. They are designed so that their endpoint (full colour change) correlates with the achievement of a specified level of microbial lethality: specifically, they are calibrated to respond equivalently to a defined overkill sterilisation cycle. In practical terms, a Class 5 indicator passes only if the combination of time, temperature, and steam saturation was sufficient to achieve the required process lethality — not just to reach the target temperature.

This makes Class 5 integrators the closest equivalent to a biological indicator (a preparation of resistant bacterial spores) that a chemical indicator can achieve, and they are accepted by many healthcare systems as a routine alternative to biological indicators for parametric release decisions in steam sterilisation.

Class 5 integrators are placed inside the most challenging position within the load, typically inside a purpose-designed test pack placed at the cold spot of the steriliser. If the integrator passes and the physical cycle printout is within specification, the load can be released with high confidence.

Typical products: SteriTec Integraph — Class 5 integrating indicator for steam sterilisation cycles at 121°C, 132°C, and 134°C.

Where to use: Inside test packs placed at the most challenging position in the load. Some departments also use Class 5 integrators inside selected complex instrument sets as an enhanced assurance measure.

Class 6 — Emulating Indicators

Purpose: To confirm that a specific, defined sterilisation cycle was completed in full.

Class 6 indicators are the most cycle-specific of all the chemical indicator classes. Unlike Class 5 integrators, which are calibrated to a minimum lethal process, Class 6 emulating indicators are designed to match the exact parameters of a particular, defined sterilisation cycle — a specific combination of temperature, time, and steam quality. They are calibrated to pass only when that complete time-temperature-steam combination is achieved, and to fail if any parameter is missed.

This makes Class 6 indicators extremely useful for routine monitoring in departments running a fixed, validated cycle, because the indicator effectively confirms that the complete specified cycle was run — not just that lethal conditions were present. They are not appropriate for use across variable cycle types, as they are rated to specific cycle parameters and would not give meaningful results in a different cycle.

Typical products: SteriTec Emu-Graph 7 — Class 6 emulating indicator available for 121°C, 132°C, and 134°C steam sterilisation cycles.

Where to use: Inside instrument packs and test packs during routine production cycles, particularly in departments running a single defined standard cycle and seeking high assurance per pack.

Indicators for Non-Steam Sterilisation Processes

Not all sterilisation processes use steam. Sterile services departments using low-temperature sterilisation systems require indicators specifically designed for those processes, as standard steam indicators will not respond correctly in a different sterilisation medium.

Hydrogen Peroxide (H₂O₂) Plasma Sterilisation — STERRAD Systems

Low-temperature hydrogen peroxide gas plasma sterilisation is widely used for heat-sensitive instruments — including flexible endoscopes, laparoscopic instruments, and electronic devices — that cannot withstand the temperatures required for steam sterilisation. The STERRAD system (Advanced Sterilization Products) is the most widely used hydrogen peroxide steriliser in UK healthcare.

SteriTec Cross Checks P are chemical indicators specifically designed and calibrated for use in STERRAD hydrogen peroxide sterilisation systems. They respond to the critical variables of the H₂O₂ process and should be used inside instrument packs and test packs in place of steam indicators, which are not appropriate for this environment.

Low Temperature Steam-Formaldehyde (LTSF)

Low temperature steam-formaldehyde sterilisation uses a combination of steam and formaldehyde gas at low temperatures, and is used for heat-sensitive instruments that can tolerate some moisture. It requires indicators rated specifically for LTSF conditions.

SteriTec Cross Checks F are calibrated specifically for LTSF sterilisation cycles.

Ethylene Oxide (EO) Gas Sterilisation

Ethylene oxide sterilisation is a low-temperature process used for single-use devices and items that cannot tolerate heat or moisture. EO leaves residual gas on instruments and requires long aeration periods, which means it is less commonly used in routine hospital sterile services but remains important for certain device types.

EO-specific indicator tape is available for use with EO sterilisation processes; standard steam or hydrogen peroxide indicators must not be substituted.

Washer-Disinfector Process Indicators

Before sterilisation, reusable medical instruments must be cleaned — typically in a washer-disinfector — to remove bioburden to a level that sterilisation can be effective. The washer-disinfector process is itself a validated process with its own monitoring requirements.

SteriTec Wash Checks are process indicators designed for use in washer-disinfectors to confirm that wash and thermal disinfection conditions were met. They are not sterilisation indicators and are not classified under ISO 11140-1, but they form an essential part of the overall instrument decontamination monitoring system.

How to Choose the Right Chemical Indicator Class

The correct combination of indicator classes for your department will depend on your sterilisation processes, the guidance in HTM 01-01 (or its devolved equivalents), and your department's quality management system. As a general framework:

Every pack and every load should include a Class 1 process indicator on the outside, to distinguish processed from unprocessed items and to provide a handling and tracking reference.

Every steam steriliser, every day should be tested with a Class 2 Bowie Dick test before the first processed load, to confirm adequate air removal.

Inside packs and test packs, use Class 4 as a minimum, with Class 5 or Class 6 for enhanced assurance — particularly for complex, heavily loaded, or implant-containing loads where release decisions carry higher risk.

Non-steam processes require process-specific indicators. Never use a steam indicator in a hydrogen peroxide, EO, or LTSF steriliser.

Washer-disinfectors should be monitored with washer-disinfector-specific indicators, independently of the downstream sterilisation monitoring.

If you are designing or reviewing your monitoring programme, the primary reference documents are HTM 01-01 (England), SHTM 01-01 (Scotland), WHTMs (Wales), ISO 11140-1, and your steriliser manufacturer's validated process specification.

Frequently Asked Questions

Can a Class 5 integrator replace a biological indicator for load release? In many healthcare settings, Class 5 integrating indicators are accepted as part of parametric release protocols, alongside satisfactory physical cycle data, as an alternative to routine biological indicator testing for steam sterilisation. However, biological indicators remain required for periodic requalification and validation, and for certain high-risk loads such as implantable devices, depending on local policy and regulatory requirements. Always follow the guidance in HTM 01-01 or your relevant national guidance document.

What is the difference between a Class 5 integrator and a Class 6 emulating indicator? A Class 5 integrator is calibrated to confirm that a minimum lethal process was achieved — it integrates across a range of time-temperature combinations that meet the required lethality. A Class 6 emulating indicator is calibrated to a specific, defined cycle (e.g. 134°C for 3.5 minutes) and confirms that exact cycle was completed. Class 6 is more appropriate when a single validated cycle is run consistently; Class 5 is more appropriate where cycle parameters may vary or where a conservative minimum lethality confirmation is needed.

Do I need different indicators for 121°C and 134°C steam cycles? Yes. Class 4 and Class 6 indicators are cycle-specific and must be matched to the sterilisation temperature your steriliser runs. Using a 134°C-rated indicator in a 121°C cycle, or vice versa, will give incorrect results and should not be done.

Can I use a steam chemical indicator in my STERRAD hydrogen peroxide steriliser? No. Steam indicators are not calibrated for hydrogen peroxide processes and will not give meaningful results in a STERRAD or similar H₂O₂ system. Use only indicators specifically designed and validated for hydrogen peroxide sterilisation, such as the SteriTec Cross Checks P range.

I have a question about indicator placement or how to use a specific product in my steriliser — who should I contact? Placement requirements vary by steriliser model, load configuration, and the specific product being used, so we always recommend going directly to the source for this kind of guidance. Contact us and we will liaise with the manufacturer to get you the correct instructions for your system.

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Legal Disclaimer

The information provided in this guide is intended for general informational purposes only and does not constitute regulatory, clinical, or compliance advice. While Temperature Indicators Ltd has made every effort to ensure the accuracy of the information presented, sterilisation standards, healthcare technical memoranda, and regulatory requirements are subject to revision and may vary by devolved nation, healthcare organisation, and device type. Sterile services departments are responsible for ensuring that their monitoring programmes comply with all applicable legislation, national guidance, and their steriliser manufacturers' validated process specifications.

The selection of appropriate chemical indicators for your sterilisation processes should be carried out in consultation with a qualified decontamination practitioner, your steriliser manufacturer, and with reference to the current edition of HTM 01-01 (England), SHTM 01-01 (Scotland), or equivalent national guidance. Chemical indicators are one element of a comprehensive sterilisation assurance programme and are not a substitute for a fully validated process, correctly functioning equipment, and appropriately trained staff. Temperature Indicators Ltd accepts no liability for any loss, damage, patient harm, or regulatory non-compliance arising from reliance on the information contained in this guide.

For guidance on decontamination standards and sterile services practice, refer to the NHS Estates Health Technical Memoranda (HTM 01-01) and consult your regional decontamination lead or device manufacturer. If you have questions about indicator placement, product-specific usage instructions, or how a particular product performs in your sterilisation system, please contact us directly — we will obtain the relevant information from the manufacturer and ensure you receive accurate, system-specific guidance.

About Temperature Indicators Ltd

Temperature Indicators Ltd is a specialist global distributor solely focused on temperature-sensitive labels, tags, and indicators for cold chain monitoring, process validation, and regulatory compliance. With 35 years of experience and operations shipping to over 50 countries worldwide, we supply food manufacturers, pharmaceutical distributors, sterile services departments, and logistics providers with the temperature monitoring solutions they need to maintain compliance. Contact us for expert guidance on temperature monitoring for your application.

Frequently Asked Questions

What are the six classes of chemical indicator defined by ISO 11140-1?

ISO 11140-1 defines six classes: Class 1 (process indicators, applied to pack exteriors), Class 2 (specific test indicators, including the Bowie Dick air removal test), Class 3 (single variable indicators, responding to temperature only), Class 4 (multi-variable indicators, responding to time, temperature, and steam quality), Class 5 (integrating indicators, calibrated to confirm minimum microbial lethality), and Class 6 (emulating indicators, validated to the exact parameters of a defined sterilisation cycle).

What is the difference between a Class 4, Class 5, and Class 6 chemical indicator?

Class 4 multi-variable indicators confirm that conditions within a pack were met but are not correlated to a specific level of microbial lethality. Class 5 integrating indicators are calibrated to the performance of biological indicators and confirm that minimum sterilising conditions for the destruction of resistant spores were achieved — the most demanding chemical indicator class. Class 6 emulating indicators are validated against the exact parameters of a specific defined cycle, making them cycle-specific but not transferable between different sterilisation programs.

Can a Class 5 integrating indicator replace a biological indicator for sterilisation load release?

In many CSSD environments, Class 5 integrating indicators are accepted as the basis for routine load release decisions in place of biological indicators, subject to local policy and compliance with HTM 01-01 (England), SHTM 01-01 (Scotland), or equivalent national guidance. Biological indicators remain the primary tool for qualification, revalidation, and investigation of suspect cycles.

How often should a Bowie Dick (Class 2) test be performed?

The Bowie Dick test should be performed every day before the first sterilisation load is processed in a pre-vacuum steam steriliser. The test pack must be run alone in an empty chamber at standard cycle parameters (typically 134°C for 3.5 minutes in the UK). A failed result indicates inadequate air removal and the steriliser must be taken out of service pending investigation.

Which chemical indicators should be used for hydrogen peroxide (STERRAD) sterilisation?

Standard steam sterilisation indicators must not be used in hydrogen peroxide plasma cycles. Hydrogen peroxide processes require process-specific indicators such as SteriTec Cross Checks P, formulated to respond to the specific parameters of low-temperature hydrogen peroxide sterilisation. Using steam indicators in a STERRAD cycle will not provide valid evidence of sterilisation conditions.

Do chemical indicators confirm that instruments are sterile?

No. Chemical indicators confirm that the physical and chemical parameters of a sterilisation cycle were met during processing, but they do not confirm sterility of the processed items. Sterility is a probabilistic concept that cannot be tested directly. Chemical indicators are one component of a multi-parameter assurance system that also includes physical cycle records, biological indicator monitoring, validated packaging, and correct load configuration.

What does HTM 01-01 require for chemical indicator use in CSSD departments?

HTM 01-01 requires Class 1 process indicators on the exterior of every processed pack, Class 2 Bowie Dick testing daily before first loads in pre-vacuum sterilisers, and Class 4 or higher indicators inside every pack to verify internal conditions. For non-implant loads, Class 5 indicators may be used to support load release. Scottish departments follow the equivalent SHTM 01-01.

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