An Introduction to Steam Sterilization

Performed by supplying dry, saturated steam under pressure, moist heat sterilization (or autoclaving) is used in hospitals to sterilize the surfaces of various utensils, such as hollow items or wrapped goods. The heat from the steam condensation wraps the items in the sterilizer, simply eliminating the microorganisms. Irreversibly damaging the cells via coagulation is the result.

Objects sterilized using moist heat need to be non-heat-sensitive items, for example, reusable medical equipment, dental instruments, simple surgical instruments, textiles or surgical equipment with cavities. Moist heat sterilization takes at least three minutes at 134 °C and a pressure of 3 BAR, or at least 15 minutes at 121 °C and a pressure of 2 BAR.

Beneficial for pharmaceutical products and so on, using saturated steam to sterilize pharmaceutical equipment, products, and reagents is a widely used sterilization method, an energy-efficient process due to the latent heat released when steam condenses on the items.

Objects which are sterilized using moist heat are usually non-heat-sensitive items, e.g., reusable medical equipment, dental instruments, simple surgical instruments, textiles or surgical equipment with cavities. Making the vital vacuum can be challenging as insufficient level results in a limited capability for the steam to infiltrate cavities in instruments.

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The Steam Sterilization Process

The benefits of the steam sterilization process include being non-toxic and simple to control. The key parameters are drying, air removal, steam contact, time, moisture, temperature and pressure, the latter two being the most crucial factors. According to EN285, their measurements require an accuracy of +/- 0.5 °C or more at the sterilization temperature, and +/- 1.6% or better for the pressure over the scale range of 0 to 4 Bar.

Air Removal

Before the operation, and to secure saturated steam conditions, air must be removed from the sterilizer chamber and load, and this is done via a vacuum system that provides a series of vacuum pulses.

Complete air removal is technically impossible, but the level should be kept at an absolute minimum (high dilution factor). Sterilization failures can be due to insufficient air removal, leaks in vacuum or poor steam quality and the presence of too many non-condensable gases.

Drying

Proper and complete drying is usually performed by applying a vacuum to the chamber at the end of the cycle, boiling all condensates and transporting them away through the vacuum. The load being dry enough when leaving the sterilizer is vital, as it could lead to re-contaminated.

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Steam Contact

Ensuring enough of its stored energy is transferred to the object via condensing, the direct steam contact to the contaminated surface is very important. In contrast, the amount of energy stored in steam is a lot higher than in dry air or water at equal temperatures, which is why the preferred solution is using saturated steam.

Time

Time is a crucial factor because bacteria does not die instantly, which is why a minimum time is needed to eradicate them all. Time is therefore linked closely to temperature as the killing effect (death value/lethality value) is dependent on both.

The killing effect is shown as the lethality value, which should reach an equal value by sterilizing at 121 °C for 15 minutes as one would by sterilizing at 134 °C for 3 minutes. Logarithmic correlation is why the same killing effect can be done by adjusting the exposure time and temperature.

A substantial amount of time can be saved by choosing 134 °C if a product can endure higher temperatures. When choosing the required lethality value, sterility assurance level (SAL) must be considered as this differs based on the application but is typically defined as sterile around 1/1,000,000. This means that only one out of a million bacteria will have survived sterilization.

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Temperature and Pressure

Lethality value and temperature are linked. Therefore, this parameter can also be employed to check the autoclave. Pressure can be converted into theoretical temperature when using moist heat sterilizers, which can then be compared to the actual temperature to assess whether the steam is saturated, eliminating the risk of air pockets that could threaten the process.

Moisture

Crucial and clean steam moisture has a high impact on destroying proteins by denaturation (coagulation). Superheated steam (above its saturation temperature) must be avoided as it will not have enough moisture for proper sterilization.

Documentation Requirements

Daily Routine Control

To control the sterilization process as a daily routine control of air removal, autoclaves that make use of vacuum chemical and biological indicators are used in the hospital central sterilization service departments (CSSD).

Only highly subjective results are created with these indicators, and purchase and running can be costly. Due to this, alternative electronic devices have been available, permitting the validation of autoclaves or washers/disinfectors as well as daily routine controls. The 2017 sterilization guidelines that were officially developed by experts admitted the possibility that these devices measure and assess physical parameters and, based on the findings, enable the parametric release of the loads.

Based on the measurements of critical physical parameters by an independent electronic device, parametric release provides an accurate and quick release compared to other biological tests. Test results are available immediately after the process has finished and data is read, while biological tests require long-term incubation. This is vital in the case of urgent orders involving the sterilization of surgical sets.

Regular Qualification and Validation

Further to the daily routine control of sterilizers, all sterilizers need regular qualifications and validations as a regulatory requirement, which are dependent on the use and the country. Standardized cycles are recommended with sterilization temperatures of 134 °C for 3 minutes or 121 °C for 15 minutes in Europe.

Several other measurements could be considered critical as well as achieving minimum and maximum temperatures for set periods, including equilibration time, which is the difference between the first sensor reaching set temperature and the last sensor to do so – as well as the spread of temperatures during sterilization and the deviation of individual temperature sensors during the sterilization period.

Calibration of temperature sensors is vital factor when qualifying steam sterilizers. Sensors being within an acceptable accuracy (at least 0.5 °C) is very important, meaning calibrating prior to the validation run, known as the pre-calibration process.

The accuracy is checked to make sure that the sensors are still within the predefined tolerances, after validation, and this process is known as post-calibration or verification.

If this process shows that the accuracy was not within the required parameters, it is considered a fail and must be repeated after correction.

Several leak tests are required before and after mounting the feed on the autoclave when working with wired systems. Checking the sterilizer for possible leaks is vital and if air can penetrate and enter the chamber during operation, the insurance of having saturated steam available is completely lost, which is why a leak test is required.

Using the Appropriate Equipment

Versatile and reliable equipment for patient safety is important to provide smoothness and compliancy and the differences between various equipment and methods can be detrimental. Many pieces of equipment are deemed acceptable for routine control and/or validation of steam sterilization, each with positives and negatives to consider.

Daily Routine Control

Some devices perform routine and batch control to eradicate the requirement for both chemical and biological indicators. For objective results and efficiency, electronic devices are recommended for Bowie Dick testing.

Auditable reports provide a clear-cut view of the process and the result is essential. Selecting an electronic Bowie Dick test device that can run several cycles in a sequence is advantageous as simple models have to cool down for 90 minutes before use, leading to downtime or expensive backup devices.

Regular Qualification and Validation

Wireless loggers are more accurate, stable and repeatable due to RTD sensor design, drifting far less over time. In Europe, a minimum of 12 measuring points are required for a single sterilizer validation run with a volume of less than 2 m3. Wired cable systems may be cheaper to begin with but, from a monetary viewpoint, they need a lot more resources.

Saving user resources over time, a wireless data logging system is a larger initial investment, but faster to operate. The cables need a feed-through setup for the thermocouples to access the chamber, which requires additional resources and introduces a leakage risk.

Image Credit: Ellab

About Ellab

Ellab Validation & Monitoring Solutions - Industry Leading Accuracy

Ellab has been your validation and monitoring partner since 1949, offering wireless data loggers and thermocouple systems for thermal validation processes as well as environmental wireless monitoring solutions.

We serve both small and large companies within the Life Science and Food industries and have solutions for almost all applications such as sterilization, freeze drying, environmental chamber testing, depyrogenation, warehouse mapping, pasteurization and many more.

In 2019 we joined forces with Hanwell Solutions Ltd. in UK in order to expand our product offering to cover industry leading wireless environmental monitoring systems.


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Last updated: Jan 15, 2020 at 4:51 AM

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