How to Validate an Autoclave: Sterilization Cycle Development

Cycle development is an important part of both the autoclave procurement and validation processes.

In previous posts about validation, we explored Installation Qualification (IQ), Operation Qualification (OQ), and Performance Qualification (PQ). We also discussed the importance of steam quality testing as part of validating your autoclave, as well as Factory Acceptance Testing.

In this post, we provide guidance in developing sterilization cycles and specifically to answer these questions about cycle development:

• When should I start thinking about cycle development?
• How do I determine the proper cycle type and cycle parameters needed to render my load sterile?
• How do I choose a sterilization process that does not destroy my load?
• How do I validate the sterility of my load?

It’s worth pointing out that in most cases (e.g. a general laboratory setting) cycle development isn’t required or has already been performed by others in the lab. Cycle development is most commonly used for biotechnology, pharmaceutical, or medical device companies that are regularly validating their autoclaves and/or sterilizing new or unique loads.

The Purpose of Cycle Development

Cycle development is performed to provide you with assurance that the load you are sterilizing is, in fact, sterilized reliably every time. It provides you with a sterilization process that can be validated. Ideally, cycle development should begin before the sterilizer is purchased, since the cycle types will affect the type of autoclave you need, and changes in the field after installation are generally difficult and expensive. At the very least, cycles should be developed before performing a full validation of the autoclave and, more specifically, before Performance Qualification (PQ) validation.

A Brief Overview of Steam Sterilization Cycles

It goes without saying that not all loads can be run with the same sterilization cycle. This is why autoclaves are equipped to run various cycle types. Unfortunately, choosing a cycle type can be confusing, not to mention the confusion around selecting the proper sterilization time and temperature. The following is a list of the most typical steam sterilization cycles used in laboratories, healthcare, and pharmaceutical production.

Gravity and Liquids

Gravity and Liquids cycles both preheat the load (through gravity displacement), raise the temperature by admitting steam to the chamber, and hold the temperature constant (more or less) throughout the exposure phase of the cycle. The difference between the two is in what happens after the Sterilization phase. Gravity cycles vent steam rapidly to the drain and may evacuate the chamber to dry the load. Liquids cycles slowly drop the pressure and temperature to ensure that the liquid load does not boil over as it cools. Liquids cycles may use a load probe to measure and control the Sterilization phase of the cycle from within the actual liquid load.

Vacuum

This cycle type is similar to a Gravity cycle, except for what happens before the Sterilization phase. Multiple pulses of vacuum followed by steam injection are used to remove air from the chamber and the load. This is the cycle type of choice for cannulated instruments, tubing, plumbing, animal bedding, and anything complex that presents a challenge to steam getting to the surfaces to be sterilized. It is never to be used with liquid loads.

Air-Over-Pressure

This cycle type is similar to a Liquids cycle except that it injects compressed air into the chamber at the Sterilization phase to maintain pressure during cool down. This cycle type is best used for small-volumes of liquid loads that tend to boil over.

To get an overview of steam sterilization, check out our video here:

Developing a Sterilization Cycle

Step 1: Determine the Cycle Type

Which cycle do you use? Refer to the flow-chart below to help you determine which cycle type is best for your load.

Step 2: Determine the Cycle Parameters: Time and Temperature

Once the cycle type is determined you must decide on the key cycle parameters: sterilization temperature and sterilization time. The vast majority of loads sterilized in the laboratory setting are sterilized at 121.1°C/250°F for 30-90 minutes.

If your load contains items that are sensitive to heat, the default cycle temperature of 121.1°C/250°F may be too hot, and you might need to consider a lower temperature. Using lower sterilization temperatures will result in longer sterilization times. This can be calculated by using the F0 relationship:

F0 is the accumulated lethality in minutes at 121.1°C/250°F.
t = the amount of time in minutes of the sterilization phase of the sterilization cycle.
T = the sterilization temperature in °C.
The F0 equation solves to:

Let’s take an example by running the numbers for a sterilization cycle using 110°C as the sterilization temperature. To solve for t (time), plug in an F0 value of 15 along with the desired T (110°C) on the right side of the equation:

t = 193 minutes
As you can see, lower temperature cycles take more time to achieve sterilization. As a comparison, at 121.1°C, this cycle would theoretically take 15 minutes to achieve sterilization.

Step 3: Validate the Cycle

Once you have taken a “best guess” at your cycle type, sterilization temperature, and sterilization time, you must validate the cycle to ensure the load will be sterile. These test cycles should be loaded in a way that is representative of the actual load to be sterilized. It is recommended to run your test cycles at half of the desired sterilization time (this is referred to as the “half-cycle” method).

Sterility testing is confirmed with the use of biological indicators (BIs). BIs are placed in intimate contact with the load and are placed throughout the chamber. For dry loads, spore strips are used. For liquid loads, you either use a hermetically-sealed ampoule BI suspended in the liquid with a string or a wire, or you can inoculate the actual liquid load with a challenge organism (Geobacillus stearothermophilus) and culture it after the cycle is complete. Typically, at least 10 BIs are used per cycle. If all BIs show no growth (i.e. all the bacteria are destroyed) in three successive cycles, you have validated the cycle.

Cycle development can be an important step not only in the validation process – especially if new loads are being processed – but in the procurement phase as well. Planning ahead to ensure you have the right autoclave for your needs will save time, money, and difficulty down the road.

Contact Consolidated if you are interested in learning more about testing procedures and validation processes.

(Written by: Jonathan A. Wilder, Ph.D., Stericert div. of H & W Technology)

(Reference: ANSI/AAMI/ISO 17665-1: 2006/(R)2013 Sterilization of health care products—Moist heat—Part 1: Requirements for the development, validation, and routine control of a sterilization process for medical devices, definition 3.17.)