The Principles of Medical Device Decontamination

• Self Contained Biological Indicators (SCBIs) : Bacterial spores are impregnated onto a carrier, often a paper strip, which is then placed inside a plastic containment tube. A glass vial of nutrient broth is then added to the tube and sealed with a cap containing a filter system which allows sterilant penetration but prevents recontamination after use. The SCBI is placed within a medical device or medical device load and exposed to the sterilization process. The SCBI is then recovered, the vial of nutrient media cracked to release the liquid onto the spore strip and then incubated. Growth or no growth is indicated by a colour change in the nutrient broth. The benefit of this type of BI is that no specialist skills or aseptic manipulation is required to obtain a BI result.

Biological Indicator Read Out Time

Traditionally, and in accordance with standards and guidance, BI’s were incubated for up to seven days before a negative growth could be recorded. However, in recent years faster incubation approaches have been approved by regulatory agencies, such that BI results can be obtained within 24 hours of incubation. Even faster results can be obtained when using certain types of BI. These BIs are constructed in the form of self contained biological indicators, whereby the nutrient medium contains a special substrate which releases a fluorescent compound if living spores are present. Fluorescent compounds can be detected at extremely low concentrations by fluorimeters. Thus as soon as viable spores begin to metabolise the substrate within the growth medium to release the fluorescent compound, the incubator/reader can detect this and show a positive growth result. Such results can be obtained within 30 minutes of commencing incubation. The determination of the read out time for a BI is now described in an internationally agreed standard ISO 111388 6 .

Chemical Indicators (CIs)

CIs are monitoring devices which consist of a mixture of reactive inks printed on a substrate which respond to defined characteristics of the sterilization process so that a visible change is observed after suitable exposure. When used to monitor a sterilization process they react to specified properties of that process and provide visual evidence that certain process characteristics were achieved.

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