Sterilization indicator types and where each goes in a load

Chemical sterilization indicator types: classes 1 to 6

ISO 11140-1 groups chemical indicators into six classes. Class 1 process indicators are the tape, bag print or tag colour that separates processed from unprocessed items; they prove exposure only. Class 2 covers specific tests such as the Bowie-Dick air-removal sheet. Class 3 reacts to one critical variable, class 4 to two or more, class 5 integrating indicators track all critical variables and are designed to correlate with biological indicator performance, and class 6 emulating indicators are cycle-specific and verify the parameters of a named cycle.

In practice a CSSD uses a mix. Class 1 goes on every outside surface, class 5 or 6 goes inside every pack or container, and a class 2 test runs daily on prevacuum steam sterilizers. Always read the indicator manufacturer's statement of which sterilization process and cycle it is validated for, because a steam strip is meaningless in a plasma cycle.

Biological indicators and autoclave validation

A biological indicator carries a known population of resistant spores, typically Geobacillus stearothermophilus for steam, and is incubated after the cycle. It is the only routine tool that shows a lethality outcome, which is why autoclave validation by biological indicator remains the reference check. Rapid-readout systems shorten the wait, but the principle is unchanged.

Typical practice is a biological test at least weekly, preferably every day the steriliser is used, plus after installation, relocation, major repair and for every implant load. Local policy and your accreditation body set the exact frequency, so confirm it in your own SOP. Autoclave validation strips alone are not biological tests; paper strips with a colour change are chemical indicators.

Process challenge devices and hard-to-reach spots

A process challenge device, or PCD, holds an indicator behind a restrictive path, such as a lumen or a helix, to represent the hardest item in the load. Helix and similar devices test steam penetration into hollow instruments, which matters for laparoscopic shafts and suction tubes.

Choose the PCD that mimics your real challenge. A single standard pack challenge is not enough if the load is dominated by long lumened instruments or heavy rigid containers.

Where to place indicators in a container load

For rigid containers, put a class 5 or 6 indicator in the area the steam reaches last, usually the centre of the lowest basket and away from the filter, and keep a class 1 indicator or tamper seal on the outside. REDA sterilization containers are offered with indicator labels as an accessory, so a label can be fitted to every container and the result read at the point of use.

In a mixed load, place the biological indicator or PCD in the most difficult spot of the chamber, commonly near the drain at the bottom front, and in a representative container. Record the lot number and result against the load number.

• Outside every pack or container: class 1 indicator or seal.
• Inside every pack or container: class 5 or 6 indicator.
• Daily on prevacuum steam units: Bowie-Dick or equivalent class 2 test.
• Routine biological test: in a PCD in the most challenging position.

Common mistakes and records

The usual errors are trusting a class 1 colour change as proof of sterility, storing indicators in damp or sunlit rooms, using expired lots, and placing the test at the easy front of the shelf. Another is failing to act on a doubtful result: any failed or ambiguous indicator should quarantine the load and trigger recall steps from your SOP.

Keep indicator results with the load log so they can be traced during an audit. For container systems, see how indicators combine with filters in sterilization container systems, and read autoclave validation guidelines for the wider validation programme. Beckfinity's specialists can help you specify containers and accessories that match your CSSD's indicator policy.