Classification of chemical indicators

The old European standard EN 867-1 has been replaced by the European and International standard EN ISO 11140-1 classifying chemical indicators depending on their usage and their performance.

While the old EN 867-1 differentiated 4 classes, which were coded with A to D, the new EN ISO 11140-1 classifies 6 different indicator classes from 1-6.
The number does not allow any conclusion about the quality, e.g. an indicator class “6″ according to EN ISO 11140-1 is not “better” than an indicator class “2″, but specifies different
characteristics by application. The standard EN ISO 11140-1 contains the definitions of chemical indicators. However colloquial language is different from the definition in the standard.

The figure illustrates the different components of an indicator/indicator system.
Fig. Relation between the components of the indicator (according EN ISO 11140-1, Annex E)

An indicator is a test system in the form it is finally used for testing.
According to the standard EN ISO 11140-1 two different product categories are named indicator:
1. Indicator strips where an indicator reagent is placed on a carrier (1)
2. A process challenge device containing an indicator system inside becoming a class 2 indicator (2)

6 classes are classified:
1. Process indicators
(EN ISO 11140-1, class 1):

These indicators, e.g. autoclave tape, are fixed on the surface of the packages which are going to be sterilized. These indicators are not applicable to provide Information about the result of the sterilization process, but only document that the package has passed the sterilization process (logistic Information, no Information about sterility).

2. Indicators for use in specific tests
(EN ISO 11140-1, class 2):

Those indicators have performance requirements, which are not described in the above Standard but in other Standards, e.g. the BD-test according to EN 285 or the helix test according to EN 867-5 Hollow Load, which conforms to the gke batch monitoring System. These tests have quite often higher requirements than indicators of class 3 to 6 described later on. Those specific tests consist of a PCD (process challenge device) with a chemical indicator inserted. Their specifications are described in specific Standards. The test is carried out only in the combination using the indicator inside of the specified PCD. It does not make sense to determine the class of the indicator in the PCD since always the whole System consisting of PCD and the indicator inside is tested for Special characteristics. The specifications of class 3 to 6 indicators have completely different characteristics from those indicators placed in the PCD to form a class 2 indicator. For example class 5 and 6 indicators for steam sterilization processes are tested in dry heat where they should not change their colour to the same colour as in steam sterilization processes. They will change their colour in steam sterilization processes to pass conditions even if a high amount of non condensable gases (NCG) is present, unless NCG within the package do not separate after steam Condensed. Chemical indicators in PCDs have completely different requirements. Their colour change has to be adapted to the humidity inside of the PCD when used in steam sterilization processes. Requirements specified in class 2 indicators describe the detection of NCG which are
not specified in the requirements of class 3 to 6 indicators. The chemical indicator strips used in gke PCDs are consumables for those indicators (class 2) and have higher requirements than the indicators described in class 5 or 6. Indicators of class 5 or 6 do not have specific requirements to NCG which are necessary for those indicators which are used inside of PCDs. The indicators in those PCD Systems fulfil the specifications of class 5 or 6 but not vice versa.

3. Single variable indicators
(EN ISO 11140-1, class 3):
To monitor one parameter, e.g. an achieved temperature. These indicators are used only very seldom in sterilization processes.

4. Multi-variable indicators
(EN ISO class 4):

These indicators monitor two or more critical parameters relevant for the sterilization process, e.g. temperature over time. They are not sufficient to monitor the efficacy of most sterilization processes except dry heat and radiation sterilization processes because most other sterilization processes require to monitor the integral of the essential parameters.

5. Integrating indicators
(EN ISO 11140-1, class 5):

A multi-variable indicator should secure the whole sterilization process and give the same information as Standard biological indicators according to EN ISO 11138. All important physical and chemical specifications for the process have to be monitored.

6. Emulating indicators
(EN ISO 11140-1 class 6);

Class 6 indicators have to monitor all critical parameters of the process and are used for Special sterilization processes, e.g. higher or Iower sterilization times like the prion program of 18 min. The stated value for those indicators has to be described by the manufacturer.Especially in steam sterilization processes for class 5 and 6 indicators there exist different opinions about which parameters are critical and have to be monitored. The Standard describes in point 5.2 for steam sterilization processes that time, temperature and water are the critical parameters while NCG as critical parameters are not mentioned at all. Because NCG as a critical parameter is not mentioned in class 5 and 6 indicators for steam
sterilization processes the general requirement that class 5 and 6 indicators should monitor all critical parameters, is not achieved. As a consequence the use of class 5 and 6 indicators do not secure the sterilization efficacy of steam sterilization processes. Presently NCG can only be detected with indicators of class 2. Many users assume that class 5 and 6 indicators can successfully monitor the whole process. Due to their lack of detecting NCGs, those indicators can only guarantee sterility at the location where the indicator is placed inside of a sterilization process but they cannot secure the whole load. The use of those indicators used to be historically justified with the wrong assumption that within a sterilization process homogenous conditions would exist within the whole sterilization chamber. Today we know that NCG accumulate in such places where no remix with steam can occur while steam during condensing to water is losing most of its volume. NCG volumes below 1 ml can lead already to an insufficient sterilization process in hollow devices like minimal invasive surgical (MIS) instruments. Class 2 indicators from gke for the Bowie-Dick-Simulation-Test and batch monitoring System consist of a PCD and an integrating chemical indicator inside. They can detect extreme small amounts of NCG and secure e the penetration characteristics of steam into the most difficult internal lumens.