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Monitoring Low Temperature Formaldehyde (LTSF)
Sterilization Processes with Biological Indicators

Historically Low Temperature Steam Formaldehyde (LTSF) sterilization processes have been tested with blood-contaminated biological indicators (BIs) Geobacillus stearothermophilus according to the standard DIN 58948-14:1987. However practical experience demonstrated using BIs with blood contamination increased the resistance, but does not produce reproducible results and therefore cannot be used.

Therefore, the replacing European and International standard EN ISO 11138-5 recommends BIs without blood contamination. The resistance of the indicators should be determined in a Resistometer with a gas atmosphere created by a formalin solution, characteristic for LTSF sterilization processes. A publication of the gke application laboratory demonstrated that this kind of resistance determination can cause big failures (see Gömann, J., Kaiser, U., Menzel, R.: Reaction Kinetics of LTSF- Sterilization Processes. Central Steril. 2000, 8 (5): 290-293. These test conditions are not reproducible to determine the resistance of indicators. Therefore the publication proposed to test instead in the gas phase in a formalin solution and directly afterwards to neutralize the absorbed formaldehyde adhering on the indicator. This publication is available on the gke website (www.gke.eu)..

The Standard EN 866-5 has been replaced by the standard EN ISO 11138-5:2005 requesting Geob. stearothermophilus spore strips with a minimum population of 105 CFU/strip. The resistance has to be determined in 1-M formalin solution at 60°C. The D60°C-value has to be at least 6 minutes, the draft standard EN 15424:2007 for the validation of LTSF-processes requires an F(Bio)-value of 30-36 minutes at 60°C.For sterilization monitoring the indicators are placed inside the load at locations difficult to penetrate. If hollow devices are sterilized, the use of a process challenge device (PCD) according EN 867-5 is mandatory. This standard will be replaced by EN ISO 11140-6. After sterilization the BI has to be removed from the LTSF sterilization process and incubated as follows:

1.To inactivate remaining absorbed formaldehyde, the spore strips have to be aseptically transferred in a test tube with 10 ml sterile filtrated 2 % Na2SO3-H2O solution, briefly shaken and left at room temperature for 10 minutes.

2. Afterwards the spore strips have to be aseptically transferred in a CaSO-liquid medium and heat shocked in a tightly closed test tube at 90°C for 60 minutes.

3. Using this growth medium, the spore strips have to be incubated afterwards for at least 5 days at 55-58°C as usually and have to be checked for growth every day.For further details about resistance determination with sodium sulphite please checkabove mentioned publication and/or the European and International standard EN ISO 11138-5.

To circumvent the complex neutralization with Na2SO3 and heat shock treatment afterwards,gkeoffers a self-contained biological indicator (SCBI) that can be incubated by the user without a microbiological laboratory. The SCBIs for Low Temperature Steam Formaldehyde (LTSF) sterilization processes contain in the growth medium substances, decomposing remaining absorbed formaldehyde, so that the pretreatment with Na2SO3 according to EN ISO 11138-5 is not required anymore and the results can be obtained much faster. The SCBIs have also been designed to be used inside a gke Steri-Record® Bio-Compact Process Challenge Device (Bio-CPCD®) simulating a hollow load test according to EN 867-5. Furthermore the corresponding incubator is available as well.Biological indicators and accessories necessary for monitoring formaldehyde sterilization processes can be purchased from gkeunder the following article numbers:

1. Self-contained biological indicators (SCBI) and spore strips to test formaldehyde sterilization processes:

Art.-No.* Product Code Cap colour Population Quantity
Yellow 10^6 10
330-501 B-S-F-SS-10-5
(spore strip)
——— 10^5 100

2. Process Challenge Devices (PCD):

Art.-No.* Product Code PCDVersion Description
300-014 B-PM-OCPD-4 Oval Complies with Hollow Load
300-019 B-PM-RCPD-4 Round T Test according to EN 867-5

3. Incubator:

Art.-No.* Product Code Description/Application
610-110 I-57 Incubator with incubation temperature of 57°C to incubate G. Stearothermophilusbiological indicators
610-113 I-AB-MBP Aluminum block for incubator for all gke Steri-Record® SCBIs