Sterilisation Quality Control Indicators
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Direct answer
Sterilisation quality control indicators confirm that a cycle actually achieved the lethal condition inside the load, not just that the machine ran. Biological indicators carry a standard spore population, such as Geobacillus stearothermophilus for moist heat, and are the only method that shows a lethal effect. Chemical and physical indicators give faster supporting evidence on each load, but they do not prove that spores were killed.
What you must remember
- Biological indicator: a standard population of about 10^6 spores on a strip or in a vial, exposed in the cycle and then incubated.
- Moist heat test spore: Geobacillus stearothermophilus, incubated at 55 to 60 °C after exposure.
- Dry heat and ethylene oxide spore: Bacillus atrophaeus, incubated at about 30 to 37 °C.
- Radiation test spore: Bacillus pumilus, used to validate gamma and electron-beam sterilisation.
- Class 1 chemical indicator: a process tape that changes colour on exposure to heat and steam, proving exposure rather than spore kill.
- Bowie-Dick test: an air removal test in pre-vacuum autoclaves, run on an empty pack to check steam penetration.
- Physical indicator: gauges and charts recording temperature, pressure and time, which confirm conditions but not lethality.
- Reading results: spore growth after incubation means the cycle failed, while no growth means the lethal conditions were achieved.
Matching the indicator to the method
Each sterilisation method has its own test organism, so a spore strip valid for steam is not valid for dry heat. Incubation conditions also differ, because each organism grows best at a different temperature. The table lists the standard pairing for the common methods.
| Method | Biological indicator | Incubation | Chemical check |
|---|---|---|---|
| Steam (autoclave) | Geobacillus stearothermophilus | 55 to 60 °C | Class 1 tape, Bowie-Dick test |
| Dry heat (hot-air oven) | Bacillus atrophaeus | 30 to 37 °C | Class 1 indicator |
| Ethylene oxide | Bacillus atrophaeus | 30 to 37 °C | Class 1 or class 5 indicator |
| Gamma radiation | Bacillus pumilus | 30 to 37 °C | Dosimetry |
Steam and dry heat use different organisms because the spores respond differently to each agent. Radiation processes depend mainly on dosimetry, with biological indicators used during validation.
Results from each indicator are read against its own standard. A spore strip that shows growth after a steam cycle points to a failed cycle or a faulty load, and the affected items must be recalled and reprocessed. Routine monitoring therefore combines all three indicator types, because each one answers a different question about the cycle.
Where the exam tries to catch you
Indicator questions test which organism matches which method and what a positive or negative result means.
- Trap: a chemical indicator colour change proves spores are dead Correct: a chemical indicator shows that conditions were reached, not that spores were killed; only a biological indicator confirms lethality.
- Trap: dry heat is validated with Geobacillus stearothermophilus Correct: dry heat uses Bacillus atrophaeus, the organism matched to that method.
- Trap: growth on a spore strip means the cycle succeeded Correct: spore growth after exposure means the lethal conditions were not achieved, so the load is not sterile.
Frequently asked questions
Which spore is used to test an autoclave?
Geobacillus stearothermophilus is the standard biological indicator for moist heat, including autoclave cycles. Its spores are incubated at 55 to 60 °C after exposure, and no growth shows that the steam cycle was lethal.
What is a Bowie-Dick test and when is it performed?
The Bowie-Dick test checks air removal and steam penetration in a pre-vacuum autoclave. It is run on an empty pack, commonly at the start of each day, and a uniform colour change on the test sheet shows that air was removed.
Why does a chemical indicator not prove that a load is sterile?
A chemical indicator changes colour after exposure to heat or steam, but it does not measure spore kill. Only a biological indicator, with a standard spore population, confirms that the lethal effect was achieved.
Which test organism is used to validate gamma radiation sterilisation?
Bacillus pumilus is the standard test organism for validating gamma and electron-beam sterilisation. Radiation processes also rely on dosimetry, which measures the absorbed dose directly.
Which spore is used for dry heat and ethylene oxide sterilisation?
Bacillus atrophaeus, formerly called Bacillus subtilis var niger, is the standard test organism for dry heat and ethylene oxide cycles. It is incubated at about 30 to 37 °C, which differs from the 55 to 60 °C used for steam spores.