Alternate Approach for Defining Worst Case in Steam Sterilization Validation of Porous/Hard Goods Loads

The Test Equipment (Sterilizer)

All the runs were performed in a Fedegari Autoclave code number NA1861AS. The autoclave had a double door design. The chamber volume of the autoclave was 1075 L.

The probes used for temperature measurement/control: PT 100 (Elsi srl)

The probes used for pressure measurement/control: TP 1 (Baumer)

The external thermocouples (TCs) used for temperature measurement: all temperature measurements in the autoclave were performed by placing type “T” TCs in the load items. All TCs were calibrated to have ±0.5°C accuracy. All temperature data was logged by using the Yokogawa data logger (model GP10, Serial number: S5T209668).

Test Items (Load)

The autoclave loads used for demonstration were as follows.

1. Load Type 1: Garment load

   • The maximum for the Garment Load will be referred to as “Max Load 1”

   • The minimum for the Garment load will be referred to as “Min Load 1”

2. Load Type 2: Mixed load (including filters connected to pressure vessels, silicon tubes, etc.)

   • The maximum for the mixed load will be referred to as “Max Load 2”

   • The minimum for mixed load will be referred to as “Min Load 2”

The load items for these loads are detailed in Tables I⇓⇓⇓⇓–IV. The autoclave loads followed the fixed item fixed position approach. Every item had a defined position on the tray/stand in the autoclave. The required items were wrapped in bacterial barrier bags to ensure steam penetration. The items were oriented for maximum steam exposure and to facilitate quick condensate removal. The TCs were placed in the test items in multiple locations to map temperature. The TCs were placed such that the tips of the TCs did not contact the surfaces of the items. Also, the TCs were oriented to avoid condensate accumulation on the tips. Additionally, it was ensured that the TCs did not interfere with the integrity of the wrapping materials or inhibit the flow of steam inside the test items.

Test Design

The loads were sterilized using the following control strategy:

1. Prepulsing with a predefined pressure end point, positive pressure gradient, and negative pressure gradient, i.e., the increase and decrease of pressure was controlled by ramping/dropping the pressure at a defined rate (in mbar/sec). There was a defined vacuum hold after every pulse.

2. Heating with a predefined positive pressure gradient (in mbar/sec) to achieve the desired temperature set point.

This controlled ramping/dropping of the pressure and temperature during the prepulsing and heating phases was achieved by automated modulating action of the steam supply valve to maintain the desired set points.

The rationale for selecting the control strategy was as follows:

1. The rate of steam addition and vacuum drawing was defined. Hence, the time for the steam injection and vacuum drawing were expected to be same theoretically. Additional vacuum hold after every pulse was defined. This provided an adequate time for air removal and preconditioning of the load.

2. The rate of steam addition was defined during the heating phase prior to the sterilization hold phase. Hence, the items were expected to get the same heat input irrespective of the load size.

The recipe used for the control strategy is detailed in Table V and Table VI. Three runs of the maximum load and three runs of the minimum load were executed for both load types. Biological indicators were not used for performing the runs executed in this study. These shall be used for actual validation runs of each load, as per respective protocols and guidelines (3) (4) (5).

Recommendation 1

1. For the prepulsing phase: use a modulated rate for steam injection and a modulated vacuum rate for air removal. An additional vacuum hold step after every pulse was recommended so that the items in the autoclave will get adequate time for air removal irrespective of the load size.

Recommendation 2

1. For the heating phase: use a modulated steam injection rate so that the articles will get the same thermal input irrespective of the load size.

Based on the data, it is recommended that this control strategy should be the preferred strategy for porous/hard goods load validation. This strategy provides enough time for air removal and heating irrespective of the load size. This will ensure that the load is equilibrated and effective sterilization is achieved because the air removal is effective. The recipe/cycle design should be done as per the following chronology:

Step 1: Run the maximum load development cycle with a modulated prepulsing and a modulated heating phase.

Step 2: Run the minimum load development cycle with the same modulated prepulsing and heating phase.

Step 3: Calculate the maximum difference in air removal time between the maximum and minimum loads. Add the time difference as the vacuum hold after each pulse. (Adding the maximum air removal time as the vacuum hold would be ideal. In this case, study one example as detailed considering the average time difference [garment load] and one example as detailed considering maximum time difference [mixed load]).

Step 4: Validate the maximum load (three consecutive successful runs) with modulated prepulsing (with vacuum hold after every pulse) and modulated heating phase.

Step 5: Perform one verification/confirmation run with the minimum load.

Loads shall be considered validated upon successful completion of the above runs with biological challenge and meeting all other sterilization acceptance criteria as per respective protocol and guidelines.