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This article is based on the practice experience gained by performing hundreds of validation test conducted on the dozen of steam sterilizer of various manufacturer. It is primarily intended that personnel who performed validation testing on steam sterilizer may benefit from these experience, and that it will assist in ensuring a high level of compliance in the validation process. Personnel unfamiliar with the steam sterilization principles may use this article as a basic training tool and it may be good refresher for more experience person.

1- Stating of cycle

t1 – start time of steam injection in to the chamber

t2 – start time of vacuum pulses, temperature reduce due to vacuum

t3- time start of plateau phase

t4- time end of plateau phase

t5- time start of drying phase / post vacuum pulses

t6- time end of drying phase /post vacuum pulses

2- end of operating cycle

Ts sterilization temperature

TB sterilization temperature band

T1 maximum difference between reference (drain) temperature and temperature in the test pack (load) during holding time

T2 maximum difference between reference (drain) temperature and temperature above test pack (load) within the first 60 s of plateau period

T3 maximum difference between reference (drain) temperature and temperature above the test pack (load) during the plateau period after the first 60 s

Note: During the plateau period the temperature measured above the test pack shall not exceed the temperature measured at the reference measurement point of the sterilizer chamber by more than 5 °C for the first 60 s and 2 °C for the remaining period.

t1 plateau period test pack within the first 60 s of plateau period

t2 equilibration time

t3 initial 60 s

t4 holding time 

Definition:

Equilibration time: Period which elapses between the attainment of the sterilization temperature at the reference measurement point and the attainment of the sterilization temperature at all points within the load

Holding time: Period for which the temperatures at the reference measurement point and at all points within the load are continuously within the sterilization temperature band

Plateau period: Equilibration time plus the holding time

Sterilization: Validated process used to render a product free from viable microorganisms

Steam Quality:

Pure steam is one of the essential utility required for steam sterilization. Therefore it is important to ensure the quality of the steam is adequate. Dryness, superheat and non-condensable gases are the three important physical tests need to be conduct before proceeding for qualification cycle. Condensate quality for trace metal analysis.

Dryness: A continuous supply of saturated steam is required for steam sterilization. Excess moisture carried in suspension can cause damp loads, while too little cannot prevent the steam from becoming superheated during expansion into the sterilizer chamber.

• >90% for normal porous loads
• >95% for metal loads.

Superheat

•  <25°C of superheat

Note: Carry out an operating cycle with the sterilizer chamber empty. Place the full load, textiles in the usable space and within 5 min carry out a further operating cycle. At the end of the operating cycle check the temperature recordings to confirm that the temperature measured in the steam pipe did not differ by more than 3 K from that measured in the steam pipe during the steam quality, dryness test.

Non Condensable Gases (NCG): The steam quality test, non-condensable gases, is used to demonstrate that the level of non-condensable gases contained in the steam will not prevent the attainment of sterilization conditions in any part of the sterilizer load.

•  <3.5% NCG

Note: Test to be conducted with the empty chamber sterilization load. Carry out at least 3 tests. The maximum result shall comply with the requirements specified.

Condensate Quality

• WFI quality when condensed

Acceptance criteria:

During the plateau period the temperature measured above the load does not exceed the temperature measured in the active chamber discharge by more than 5°C for the first 60 s and 2°C for the remaining period;

The equilibration time determined from the measured temperatures does not exceed 15 seconds for chambers up to 800 L and 30 seconds for larger chambers;

The holding time determined from the measured temperatures is not less than that specified for the sterilisation temperature;

During the holding time the temperatures measured in the active chamber discharge and in the load:

• are within the appropriate sterilization temperature band
• do not fluctuate by more than ±1°C;
• do not differ from one another by more than 2°C

During the holding time:

• the indicated and recorded chamber temperatures are within 1ºC of the temperature measured in the active chamber discharge;
• the indicated and recorded chamber pressures are within 0.05 bar of the measured pressure

Biological indicators of sterilisation

Steam sterilisation. The use of biological indicators intended for steam sterilisation is recommended for the validation of sterilisation cycles. Spores of Bacillus stearothermophilus (for example, ATCC 7953, NCTC 10007, NCIMB 8157 or CIP 52.81) are recommended. The number of viable spores exceeds 5 × 10⁵ per carrier. The D-value at 121 °C exceeds 1.5 min.

Reference:

BS EN 285:2015 ; Sterilization — Steam sterilizers — Large sterilizers

Validation and Control of Porous Load Sterilisation ; Gerard Sheridan ; https://www.hpra.ie/docs/default-source/default-document-library/7_autoclave-validation_gerard-sheridan.pdf?sfvrsn