The microbial inactivation type test is a critical performance qualification for ethylene oxide sterilizers, particularly for Class B equipment. This half‑cycle method uses process challenge devices (PCDs) and biological indicators to verify that the sterilizer can achieve a sterility assurance level (SAL) of 10⁻⁶ under worst‑case conditions. The procedure described here aligns with the validation requirements for industrial ETO sterilizers designed for medical device processing.

Required Equipment and Materials

The following items are necessary to perform the type test correctly:

• Process Challenge Devices (PCDs): Four identical units. Each PCD consists of a stainless steel tube and a semi‑closed chamber that holds the biological indicator. The chamber lid or opening uses an O‑ring seal and threaded connection to ensure the only gas pathway is through the tube length. The tube has a nominal length of 4.55 m, an inner diameter of 3.0 mm (length‑to‑diameter ratio 1500:1), giving a total internal volume of approximately 32 mL, of which the chamber volume is 0.85 mL.
• Pouches: Eight paper‑plastic pouches conforming to GB/T 19633.1, GB/T 19633.2, YY/T 0698.4, and YY/T 0698.5.
• Biological Indicators: Spore strips or self‑contained indicators complying with GB 18281.1 and GB 18281.2, jellemzően Bacillus atrophaeus spores for EO sterilization.
• Recovery Medium: Appropriate growth medium specified by the biological indicator manufacturer.
• Incubator: Calibrated to the temperature required for the biological indicator.

Test Procedure (Half‑Cycle Method)

Follow these steps systematically. All biological indicators must be from the same batch, and a positive control must be included in each test run.

Step 1 – Preparation of PCDs

Place one biological indicator inside the test chamber of each process challenge device. Double‑wrap each PCD in two paper‑plastic pouches, sealing each pouch individually with a heat sealer. The double wrapping increases the challenge for the sterilant to penetrate.

Step 2 – Conditioning

No preconditioning of the PCDs is required. Viszont, they must be kept at 20°C ±5°C for at least one hour before the test. This simulates the worst‑case cold start condition, making it more difficult for the sterilant to reach the spores.

Step 3 – Loading and Cycle Initiation

Preheat the sterilizer to the normal sterilization temperature. Load all four wrapped PCDs into the chamber and start the test cycle within 15 minutes of loading. The sterilant exposure time must be set to half of the nominal exposure time of the sterilization cycle under test. All other cycle parameters (hőmérséklet, nedvesség, nyomás) remain unchanged.

Step 4 – Post‑Cycle Handling

After the cycle ends, remove the PCDs from the sterilizer. Aseptically extract the biological indicators from the PCD chambers and transfer them to the recovery medium. Incubate according to the manufacturer’s instructions, along with a non‑sterilized positive control from the same batch.

Step 5 – Interpretation

If the positive control shows viable growth, the test is valid. All biological indicators from the half‑cycle exposure must show no growth. If any test indicator shows growth, the test fails, indicating insufficient lethality. Repeat the complete procedure two more times (three runs total). Consistent inactivation across all three runs confirms the sterilizer’s microbial performance capability.

Applicability and Limitations

This type test is mandatory for Class B sterilizers but can also be applied to Class A sterilizers with pre‑set programs. Viszont, for Class A equipment, the type test does not replace a full performance qualification (PQ) under actual load conditions. Always refer to the manufacturer’s validation master plan and relevant regulatory standards when implementing this protocol.