Chemical indicator classes 1 to 6 explained (ISO 11140-1)
What Class 1, 2, 4, 5 and 6 chemical indicators each prove, where they go in a steam, EO or plasma load, and how CSSD teams choose between them.
A chemical indicator changes colour when it has been exposed to one or more conditions of a sterilization cycle. It does not prove a load is sterile — only a validated process, routine physical monitoring and, where required, biological indicators do that — but it is the quickest evidence a CSSD team has that each pack actually saw the process. ISO 11140-1 sorts indicators into six classes by what they respond to.
The six classes at a glance
| Class | Type | What it tells you | Typical use |
|---|---|---|---|
| 1 | Process indicator | The item has been through a sterilization process — it separates processed from unprocessed packs. | Indicator tape and printed indicators on reels and pouches, outside each pack. |
| 2 | Specific-test indicator | A defined test was passed — for steam, air removal and steam penetration in a pre-vacuum sterilizer. | The daily Bowie-Dick test, run in an empty chamber before the first load. |
| 3 | Single-variable indicator | One critical variable (usually temperature) reached its stated value. | Rarely specified today; largely replaced by Class 4–6. |
| 4 | Multi-variable indicator | Two or more critical variables reached their stated values. | In-pack strips for steam, H₂O₂ plasma or EO cycles. |
| 5 | Integrating indicator | All critical variables, with performance correlated to a biological indicator. | In-pack monitoring and load release for steam. |
| 6 | Emulating indicator | All critical variables of one specific cycle (for example 134 °C for 3.5 minutes). | In-pack monitoring where the cycle is fixed and known. |
Outside the pack: Class 1
Every pack leaving the CSSD should carry an external process indicator, so a nurse in theatre can see at a glance that it was processed. On wrapped packs this is indicator tape; on reels and pouches the indicator is usually printed on the material. The colour change only confirms exposure — it says nothing about conditions inside the pack.
Choose the tape for the sterilant: steam indicator tape for 121–134 °C cycles, EO indicator tape for ethylene-oxide cycles and plasma indicator tape for low-temperature H₂O₂ cycles.
Every morning: Class 2, the Bowie-Dick test
Pre-vacuum steam sterilizers depend on removing air from the chamber and the load. Residual air or a leak creates cold spots where steam cannot reach. A Bowie-Dick test pack is run in an empty chamber, usually at 134 °C for 3.5 minutes, before the first load of the day. A uniform colour change means air removal is adequate; an uneven one means the sterilizer should not be used until it has been checked.
The Bowie-Dick test applies to pre-vacuum steam sterilizers only. EO and plasma sterilizers have their own routine tests specified by the manufacturer.
Inside the pack: Class 4, 5 or 6
In-pack indicators sit at the point inside the pack that is hardest for the sterilant to reach — in the centre of a tray or between layers of textiles — and are read when the pack is opened.
- Class 4 responds to two or more variables. It is the common choice for low-temperature processes, where plasma indicator strips give a clear colour change on H₂O₂ cycles.
- Class 5 integrators respond to all critical steam variables and are designed to correlate with biological indicator performance, which is why many CSSDs use them for steam load release.
- Class 6 emulators are made for one specific cycle and suit departments that run the same validated cycle every time.
SteriCraft® steam indicator strips are available in Class 4, 5 and 6, so one supplier can cover tape, in-pack strips and the daily test.
A simple decision guide
- What is the sterilant? Steam, EO or H₂O₂ plasma — indicators are specific to the process and are not interchangeable.
- What are you checking? Outside each pack (Class 1), the sterilizer each morning (Class 2), or conditions inside the pack (Class 4–6).
- Is the cycle fixed? If every steam load runs the same validated cycle, a Class 6 emulator is precise; if cycles vary, a Class 5 integrator is the safer choice.
- What does your policy require? Follow the facility's validation and infection-control policy, and record every result with the load number.
Storage and reading
Store indicators in the conditions printed on the box, away from heat, light and chemical vapours, and use them within their shelf life. Read the result in good light against the reference colour on the product, and never use an indicator twice. If a result is unclear, treat the pack as failed and reprocess it.
Not sure which indicator fits your sterilizer? The FAQ has a short indicator chooser, or ask the desk and include the sterilizer model and cycle parameters.