MDRAO Sterilization Methods and Monitoring 2 — Questions and Answers

Question 1: What is the minimum exposure time and temperature required for a gravity displacement steam sterilizer (autoclave) cycle for wrapped instruments?

121°C for 15 minutes (Correct answer)
132°C for 4 minutes
134°C for 3 minutes
115°C for 30 minutes

Correct answer: 121°C for 15 minutes

Gravity displacement steam sterilizers require a minimum of 121°C for 15 minutes for wrapped items to achieve sterilization. Pre-vacuum cycles can use higher temperatures for shorter times.

Gravity displacement autoclaves rely on steam displacing air by gravity — steam enters at the top while air (being heavier) exits through a drain at the bottom. For wrapped instrument sets, the standard validated cycle is 121°C for a minimum of 15 minutes. Pre-vacuum (dynamic air removal) sterilizers can achieve equivalent sterilization at 132–134°C in 3–4 minutes due to more efficient air removal and steam penetration. CSA Z314 specifies that cycle parameters must be validated for each load configuration, and operators must verify that the correct cycle is selected for the type of load and packaging used.

Question 2: Which biological indicator (BI) organism is used to monitor the efficacy of steam sterilization?

Bacillus subtilis (ATCC 9372)
Geobacillus stearothermophilus (ATCC 7953) (Correct answer)
Clostridium sporogenes
Staphylococcus aureus

Correct answer: Geobacillus stearothermophilus (ATCC 7953)

Geobacillus stearothermophilus spores (ATCC 7953) are the standard biological indicator for steam sterilization due to their high resistance to moist heat.

Biological indicators for steam sterilization contain spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus), ATCC 7953. These spores are among the most heat-resistant organisms known and serve as the standard challenge organism for steam sterilization monitoring. If the sterilization process is effective, all spores are killed and the BI shows no growth. For ethylene oxide (EO) sterilization, Bacillus atrophaeus (ATCC 9372, formerly B. subtilis) is used instead. CSA Z314 requires biological monitoring with each load or at minimum weekly, with implantable devices requiring a BI in every load with quarantine until results are confirmed.

Question 3: What does a Bowie-Dick test detect in a pre-vacuum steam sterilizer?

The presence of viable microorganisms after sterilization
Air removal efficiency and steam penetration into porous loads (Correct answer)
Chemical concentration of sterilant agents
Sterility of implantable devices

Correct answer: Air removal efficiency and steam penetration into porous loads

The Bowie-Dick test specifically evaluates air removal and steam penetration in pre-vacuum sterilizers. Air pockets prevent steam contact with instruments and can cause sterilization failure.

The Bowie-Dick test (or air removal test) is run daily in pre-vacuum steam sterilizers before the first patient load to verify that the vacuum system is functioning properly and that steam can uniformly penetrate the test pack. Air residues in the chamber prevent steam from contacting all surfaces, creating 'cold spots' where sterilization may fail. The test pack contains a chemical indicator sheet; uniform colour change across the entire sheet indicates adequate air removal and steam penetration. A non-uniform result requires the sterilizer to be taken out of service, investigated, and serviced before use. The Bowie-Dick is a sterilizer performance test — not a sterility assurance test for products.

Question 4: Which type of sterilization is most appropriate for moisture-sensitive and heat-sensitive instruments such as flexible endoscopes?

Steam autoclave at 134°C
Dry heat oven at 160°C
Ethylene oxide (EO) gas or low-temperature hydrogen peroxide plasma (Correct answer)
UV irradiation

Correct answer: Ethylene oxide (EO) gas or low-temperature hydrogen peroxide plasma

Low-temperature sterilization methods such as ethylene oxide (EO) gas or hydrogen peroxide gas plasma (e.g., STERRAD) are used for moisture-sensitive and heat-sensitive instruments that cannot withstand steam autoclave temperatures.

Many modern medical devices — particularly flexible endoscopes, cameras, powered instruments, and electronic components — cannot withstand the heat and moisture of steam sterilization. These devices require low-temperature sterilization methods. Ethylene oxide (EO) is a highly effective alkylating gas that sterilizes at temperatures of 37–63°C, but requires long aeration times to remove toxic residuals and has environmental concerns. Hydrogen peroxide gas plasma systems (such as STERRAD) use low-temperature (45–55°C) vaporized H2O2 with plasma phase for rapid sterilization with no toxic residuals. CSA Z314 requires that the selected sterilization method be validated as compatible with each device's material and design.

Question 5: What is the Sterility Assurance Level (SAL) required for sterile medical devices, and what does this value represent?

SAL 10⁻³, meaning a 1 in 1,000 probability of a viable organism on a device
SAL 10⁻⁶, meaning a 1 in 1,000,000 probability of a viable organism on a device (Correct answer)
SAL 10⁻⁹, meaning a 1 in 1,000,000,000 probability
SAL 10⁻¹², meaning absolute sterility

Correct answer: SAL 10⁻⁶, meaning a 1 in 1,000,000 probability of a viable organism on a device

Medical devices require a SAL of 10⁻⁶, which means there is a probability of no more than 1 in 1,000,000 that a single viable microorganism remains on a processed device.

Sterility Assurance Level (SAL) is the probability that any single item in a sterilized batch is contaminated with a viable microorganism. The internationally accepted standard for sterile medical devices is SAL 10⁻⁶ — a theoretical probability of not more than one in one million items being non-sterile after processing. This is not an absolute guarantee of sterility but rather a statistical concept based on the inactivation kinetics of microorganisms during the sterilization process. Achieving SAL 10⁻⁶ requires validated sterilization processes, appropriate packaging, proper loading, and routine monitoring with physical, chemical, and biological indicators as specified in CSA Z314.

Question 6: Chemical indicators (CIs) used in steam sterilization are classified by class. What does a Class 5 integrating indicator measure?

Only exposure to steam (Class 1 process indicator function)
Single sterilization parameter — temperature only
All critical sterilization parameters: time, temperature, and steam quality — performing equivalently to a biological indicator (Correct answer)
Whether a package has been opened or exposed to any heat source

Correct answer: All critical sterilization parameters: time, temperature, and steam quality — performing equivalently to a biological indicator

Class 5 integrating indicators respond to all critical parameters of the sterilization cycle (time, temperature, steam presence) and are designed to correlate with biological indicator performance.

Chemical indicators are classified according to ISO 11140-1 and CSA Z314. Class 1 indicators (process indicators) simply show that a package has been exposed to a sterilization process. Class 2 are Bowie-Dick test indicators. Class 3 single-variable indicators respond to one parameter. Class 4 multi-variable indicators respond to two or more parameters. Class 5 integrating indicators are designed to respond to all critical sterilization parameters (time, temperature, and steam quality) and their performance is correlated to that of biological indicators. Class 6 emulating indicators respond to the cycle-specific critical variables of the intended sterilization cycle. Class 5 integrators are used inside packages and in the most challenging positions within loads.

MDRAO Sterilization Methods and Monitoring 2

What is the minimum exposure time and temperature required for a gravity displacement steam sterilizer (autoclave) cycle for wrapped instruments?