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Indicadores químicos: Monitoramento da esterilização a vapor
Multiple processes are available for sterilizing medical devices; however, steam sterilization remains the most widely used in healthcare settings, dental practices, and the pharmaceutical industry. Steam sterilization cycles rely on defined critical variables, primarily temperature, exposure time, and the presence of saturated steam. These parameters must be specified, validated, and continuously monitored to ensure that the conditions required for an efective sterilization are achieved and maintained.
Routine monitoring of steam sterilization cycles may be performed using biological indicators (BIs) or chemical indicators (CIs). CIs incorporate inks that undergo a defined and observable chemical or physical change when exposed to specific sterilization variables. The nature of this change is specified by the manufacturer and must be interpreted by the user as acceptable or unacceptable (fail or pass).
ISO 11140-1:2014 defines six types of CIs, intended to provide evidence of sterilization failures resulting from improper packaging, incorrect loading, or sterilizer malfunctions. It is important to note that CIs cannot be used alone to assess sterility; rather, they constitute one component of a comprehensive monitoring approach.
According to the standard, Type 1 indicators, also known as process indicators, are used externally, i.e., on the outer surface of packages or containers to be sterilized, and their sole purpose is to distinguish processed from unprocessed items. These external indicators provide evidence that an item has been exposed to the process but do not ensure that specific sterilization parameters are met. Type 2 indicators are designed for specific sterilizer performance tests, such as the Bowie–Dick test, which is intended to evaluate air removal and steam penetration in sterilizers with dynamic air removal. Type 2 indicators shall be used exclusively to monitor the specific tests for which they were designed.
Types 3 through 6 CIs are classified as internal indicators and must be placed inside sterilization packs to verify that the sterilizing agent has penetrated the packaging materials and that appropriate conditions have been met at the point of placement. Type 3 indicators respond to one critical process variable, while Type 4 indicators, also known as multivariable indicators, respond to two or more critical process variables. Type 5 CIs, referred to as integrating indicators, are designed to respond to all critical process variables and are calibrated to correlate with the performance of a BI. Type 6 indicators, known as emulating indicators, are intended to simulate specific steam sterilization cycles with well-defined parameters and are also required to
react to all process variables.
The following table summarizes the different types of CIs as defined in ISO 11140-1:
| Formato | Descrição | Uso pretendido |
|---|---|---|
| Type 1 “exposure” or process indicador | Used to distinguish processed from unprocessed items. | External indicator |
| Type 2 specific test indicators | For use in specific tests, e.g. Bowie and Dick-type test. | Sterilizer tests |
| Type 3 single variable indicator | Reacts to one critical process variable only | Internal indicator |
| Type 4 multivariable indicator | Reacts to more than one critical process variable | Internal indicator |
| Type 5 integrating indicator | Reacts to all critical process variables and is calibrated with a biological indicator (BI) | Internal indicator |
| Type 6 emulating indicator | Reacts to all critical process variables for specified sterilization processes | Internal indicator |
Many international guidelines recommend using both internal and external CIs. For instance, the World Health Organization’s Surgical Safety Checklist requires operating room personnel to verify the sterility of each instrument set before use, which can be fulfilled by inspecting both external and internal CIs.
Terragene manufactures Type 1, 2, 4, 5, and 6 indicators for monitoring several sterilization processes, including steam. These CIs are available in a variety of formats and designs. Below is a brief overview and comparison of the CIs manufactured by Terragene for steam sterilization processes, excluding Type 2
indicators, as their design, validation, and intended use involve different concepts. That is why ISO 11140 addresses Type 2 indicators in dedicated parts. Terragene manufactures the CIs in two main formats, differentiated by the physical and/or chemical mechanism through which the indicator response is expressed.
The first one includes printed-ink CIs, in which the chemical reagents are formulated into an ink and printed onto a solid substrate. Depending on the product design, the substrate may consist of a self-adhesive label, a laminated strip, or an indicator tape. When exposed to the sterilization process, the indicator ink undergoes a chemical reaction that results in a defined color change specified by the manufacturer, corresponding to the
indicator endpoint, a concept described in a later section of this document.
The second format consists of “moving front” CIs, or migrating CIs, which have a more complex construction. These indicators incorporate a paper wick and a steam and temperature sensitive chemical pellet contained in a paper/film/foil laminate. Because the ink pellet is sensitive to steam and temperature, it melts upon
exposure and migrates along the paper strip by capillary action. This migration is visible through a window and allows assessment of whether the ink reaches the ACCEPT or REJECT zones marked on the front layer of the indicator. The extent of migration depends on steam quality, temperature, and exposure time.
Accordingly, Terragene offers Type 1, Type 4, Type 5, and Type 6 chemical indicators for steam sterilization in the following formats:
Type 1 | Process Indicators
Type 1 indicators are available in printed-ink formats. Terragene manufactures these indicators as self-adhesive labels (e.g., CD23) and indicator tapes (e.g., CT22). This variety of formats allows the user to select the most appropriate option based on the nature, material, and geometry of the items to be sterilized.
Type 4 and Type 6 | Multivariable and Emulating Indicators
Type 4 and Type 6 indicators are manufactured as printed-ink strips. Available formats include strips with two reactive ink lines (Type 4; e.g., CD29), double-dot strips (Type 4; e.g., CD20), and single-dot strips (Type 6;e.g., IT27-5A).
Type 5 | Integrating Indicators
Unlike the above indicators, Terragene’s integrators are available not only as printed-ink strips (e.g., IT26-1A), but also as moving front format (e.g., IT26-C).
Key concepts for CIs: Endpoint and Stated Values
The performance of internal CIs is based on two fundamental concepts defined in the Terms and Definitions section of ISO 11140: the endpoint and the stated values (SV), which are intrinsically related. The endpoint of a CI refers to “the point of the observed change defined by the manufacturer, occurring after the indicator has been exposed to specified SV”, the SV being defined as “the minimum value or values of a critical process
variable at which the indicator is designed to reach its endpoint as defined by the manufacturer”. What does this mean in practical terms? The endpoint is the manufacturer-defined visual change that corresponds to a “pass” condition or acceptable result. In other words, the endpoint is achieved when the critical variables, or the required sterilization conditions, have been met. This change may involve a color shift or, in the case of moving-front indicators, the migration of a dye beyond an accept/reject line.
Performance requirements for Type 1, Type 4, Type 5, and Type 6 indicators
Unlike the other types of CIs, Type 1 indicators do not have manufacturer-defined Stated Values; instead, their performance is evaluated against exposure conditions specified in ISO 11140-1, which define the criteria for acceptable and unacceptable responses. In accordance with the standard, the indicator must reach its endpoint after exposure to saturated steam at 121 °C (+3/0 °C) for 10.0 minutes ± 5 s, or at 134 °C (+3/0 °C) for 2.0 minutes ± 5 s. No visible color change shall occur under reduced exposure to saturated steam or exposure to dry heat at 140 °C (+2/0 °C) for 30 minutes ± 1 minute. The dry heat test is intended to verify that process indicators for steam respond specifically to saturated steam rather than elevated temperature alone.
Regarding internal indicators, Type 4 CIs should reach their endpoint when exposed to the stated values declared by the manufacturer. Terragene produces Type 4 CIs that react to all critical process variables. For fail conditions, 11140-1 requires exposure to SVs reduced by the combined tolerances: a temperature reduction of 2 °C and a time reduction of 25%.
The requirements for Type 5 CIs in ISO 11140-1 are far more prescriptive, as their performance is linked to the inactivation of a BI compliant with ISO 11138-3. Type 5 CIs must give a “pass” result when exposed to saturated steam at 121°C for not less than 16.5 minutes, 135°C for not less than 1.2 minutes, and a third temperature and time combination specified by the manufacturer, which falls within the range 121 to 135°C. Additionally, the standard requires characterization of Type 5 CIs temperature coe cient, which serves to demonstrate that
the behavior of the CI is equivalent to the thermal resistance characteristics of the reference BI. This coe cient is derived from the slope of the curve obtained by plotting log SV and/or SV versus temperature. The indicator temperature coe cient shall be not less than 10 °C and not more than 27 °C, and the correlation coe cient of the curve established by least squares linear regression analysis of the data shall be not less than 0,9. When evaluated at SVs reduced by the combined ISO 11140-1 tolerances, the indicators are required to show a “fail” result, corresponding to a temperature decrease of 1 °C and a time reduction of 15%. The tolerances for the detection of a pass and a fail in a Type 5 CI are therefore stricter than for a Type 4.
As for Type 6 CIs, these indicators shall produce a “fail” result when tested at exposure conditions equal to the stated values minus the combined tolerances, defined as a reduction of 1 °C in temperature and 6% in exposure time. As Type 6 indicators are cycle-specific and calibrated to narrow performance windows, the tolerances for differentiating pass and fail outcomes are significantly tighter than those applicable to Type 4 or Type 5 CIs.
In compliance with ISO 11140-1, both Type 5 and Type 6 indicators for steam exposed to dry heat at 140 ºC ±2 ºC for 30 min ± 1 min shall not reach their end points. Again, the dry heat test is designed to ensure that these indicators require the presence of steam in order to respond.
The following table summarizes and compares the different SVs for steam CIs according to indicator type:
Considerações finais
CIs, together with physical monitoring and BIs, constitute the three essential technologies used for the routine monitoring of steam sterilization processes. They support effective processing by enabling the differentiation between processed and unprocessed items, demonstrating penetration of the sterilizing agent into the load, and indicating whether the required exposure conditions have been achieved at the point of placement. CIs are therefore indispensable tools for ensuring the reliability and consistency of sterilization workflows in both healthcare and industrial settings.
The selection of the appropriate type of CI is a key aspect of steam sterilization monitoring. The outside of each pack should have a Type 1 process indicator present to provide immediate evidence that a pack has been exposed. The selection of an internal CI is more complex and should take into account clinical requirements and workflow considerations. Importantly, no single type of CI is inherently superior to another. Each type is designed to fulfill a specific function and offers distinct performance characteristics. Consequently, the appropriate selection of a CI depends on the intended application, the sterilization cycle employed, applicable regulatory requirements, and the level of assurance required.
