Chemical Indicators for Monitoring Sterilization Processes: Differences Between Type 4 and Type 5 Chemical Indicators

A complete set of stainless-steel surgical instruments placed on a sterile blue drape, ready for medical use.

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Abstract /Summary

Although multiple processes can be used to sterilize single-use and reusable medical devices, steam sterilization predominates in healthcare settings. Loads of medical devices are exposed to pressurized saturated steam at a specified temperature for a defined time, known as the process variables (e.g., 132°C for 4 minutes). Sterilization processes must be specified, validated, and then routinely monitored to ensure ongoing efficacy. Sterilization processes can be monitored using physical indicators, which measure a process variable, biological indicators (BIs), which contain viable bacterial spores resistant to but killed by an effective process, or chemical indicators (CIs). The latter contain inks and dyes that undergo chemical or physical reactions during the process, giving rise to a color change specified by the manufacturer and interpreted as acceptable or unacceptable by the end user.

ISO 11140-1 specifies six types of CI. There are process indicators (Type 1) for placement on the outside of sterile packs. There are special test indicators for assessing the performance of sterilizers (Type 2; e.g., The Bowie and Dick Test). There are “internal” CIs (Types 3 to 6) placed inside each sterile pack and used to provide evidence that sterilizing conditions have been met at the point of placement.

Many national guidance documents recommend the placement of CIs both on the outside and inside of sterile packs. When implemented, the World Health Organization Surgical Safety Checklist requires operating room staff to “confirm the sterility” of every instrument set before use. This can be achieved by examination of CIs placed on both the inside and outside of the packs. Practitioners have a wide choice when deciding which type of CI to use, of which Types 4 and 5 are most common.

Type 4 CIs are required to react to two or more process variables. The manufacturer must state which process variables they react to and the exposure conditions (their stated values) that will give rise to a specified color change (endpoint). The process variables need not have a relationship to the steam sterilization process employed in the setting where they might be used. When exposed to their stated values, the CI must show a “pass” result as defined by the manufacturer. When tested for the ability to detect failures, the exposure conditions are based on the stated values for temperature being reduced by 2°C and for time by 25%. When tested under these conditions, the CI must show a “fail” result.

Type 5 CIs are required by ISO 11140-1 to give a response that relates to the inactivation of a BI and must therefore have minimum exposure conditions giving rise to a “pass” result. They must react to the three process variables for a steam sterilization process (i.e., temperature, time, and the presence of saturated steam). 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. When tested for the ability to detect failures, the three exposure conditions are based on the stated values for temperature being reduced by 1°C and for time by 15%. The tolerances for the detection of a pass and fail in a Type 5 CI are therefore tighter than for a Type 4.

Because Type 4 CIs rely on a color change in the ink, they can be harder to interpret, particularly if the starting color is similar to the endpoint color. Because a dark dye migrates along a paper wick, eventually passing an accept/reject line during a sterilization process, moving front Type 5 CIs are easier to interpret by the user. In all cases, the end user is advised to seek evidence that the variability of the response is small so that reliable results are routinely attained.

Sterilization Processes for Medical Devices

There are many processes used for the sterilization of single-use and reusable medical devices.¹ In industrial settings, physical processes such as irradiation with gamma, e-beam, or “soft” X-rays and chemical processes employing ethylene oxide gas are extensively used on a large scale, processing millions of medical devices every year. In health care settings, such as hospital sterile processing departments (SPD) or dental surgeries and general practitioners’ offices, steam sterilization predominates, representing over 90% of processes carried out.

Monitoring the Sterilization Process

Whichever and wherever a sterilization process is used, it must be specified, validated, and routinely monitored to ensure ongoing efficacy.² The characteristics of the process that must be monitored are those that kill any microbial contaminants present on the medical device; these are called the process variables.² In steam sterilization, the characteristics of the process that kills microbes are the temperature and time of exposure in the presence of moisture, usually saturated steam.³ The steam sterilization process takes place at high temperatures (e.g., 132°C), and so a pressurized saturated steam environment in a sealed pressure vessel called an autoclave is used. However, the presence of residual air resulting from an inadequate air removal stage in the process or the presence of contaminating gases in the steam supply (called non-condensable gases) can lead to inadequate moisture levels at the surfaces needing to be sterilized, thereby compromising the efficacy of the process. Any monitoring protocol must not only respond to the process variables when present at acceptable levels but also respond to inadequate levels that will compromise a process, for example, failure to attain sterilizing temperatures.

There are three basic methods for monitoring sterilization processes. There are physical indicators that measure a physical characteristic of the process (e.g., temperature). There are biological indicators (BI) containing bacterial spores that are resistant to, but inactivated by, an effective process.⁴ There are chemical indicators (CI) which consist of mixtures of reactive inks or dyes that respond to defined characteristics of the process so that a visible change is observed after suitable exposure.⁵

Chemical Indicators for monitoring steam sterilization

Table 1 shows the six types of CIs described in International Standard ISO 11140 part 1.⁵ The table shows the type (previously called class), its descriptive name, and its intended use. The standard also specifies the performance requirements for each type of CI used in different sterilization processes.

Table 1. The six types of CI described in ISO 11140-1:2014.

Format of Chemical Indicators

There are two basic formats of chemical indicators. In the first, chemical reagents are formulated into an ink that can be printed on a substrate. The substrate may be an adhesive label, sterile packaging bags, and pouches or adhesive tapes to make indicator tape.⁸ The ink changes color during the sterilization process, turning from light to dark or one distinct color to another. The choice of color change is vital when considering the use and interpretation. Those CIs that use indistinct color changes are often difficult to interpret. Most Type 4 CIs are supplied in this format.

The second format of CI is called a “moving front” CI,⁹ which has a more complex construction (Figure 1). They consist of a pellet of dye, which is placed within an indentation on a foil base. A paper wick is then placed in contact with the dye pellet, and the whole assembly is sandwiched between a semi-permeable membrane and the foil base. A label is then placed over the semi-permeable membrane with information printed on it. When exposed to the sterilization process, the dye pellet melts and begins to move along the paper strip by capillary action. The speed of migration depends on the temperature, and the distance of ink travel depends on the exposure time. These types of indicators are very easy to interpret since it is immediately obvious whether the ink has migrated beyond the accept/reject line. Ideally, Type 5 CIs should be in this format.

Figure 1. The construction of a moving front chemical indicator showing an aluminum platen (black line), an indicator dye pellet (blue oval), a paper wick (red line), a semipermeable membrane (blue line), and a paper label showing graphics (dark blue line).

Terminology used with Chemical Indicators: Endpoint and Stated values

There are two fundamental terms associated with the performance of CIs. The first is the term “endpoint,” the second is “stated value” (SV) and both are interrelated. The “endpoint” of a CI is the description of a pass or accept result specified by the manufacturer. This can be a change from light to dark or vice versa, one color to another, or the passage of a dye beyond an accept/reject line marked on the indicator. The exposure conditions that give rise to the “pass” color change or endpoint are called the “stated values,” and there may be one or multiple SVs for each of the process variables the CI responds to, depending on which type of indicator is in use. Table 2 illustrates the SVs that give rise to the endpoint for Type 1, 4, and 5 CIs.

Table 2. Examples of unexposed Type 1, 4, and 5 CIs and their appearance when reaching their endpoint after exposure to the specified stated values.

Type 4 versus Type 5 Chemical Indicators

The SPD and theatre teams have many choices when deciding which CI to use, including whether to use a Type 4 or Type 5 CI inside the pack and a Type 1 on the outside of each pack. Typically, Type 4 CIs are presented as a printed ink patch on a substrate that changes color during the sterilization process. Type 5 CIs can exist as either a printed ink patch that changes color or as a “moving front” presentation in which a colored dye migrates along a wick as it is exposed to the sterilization process reaching and passing a marker point that indicates satisfactory processing.

According to ISO 11140-1,⁵ Type 4 CIs are those that respond to two or more of the process variables required for a successful sterilization process. Type 5 CIs must respond to all process variables in a manner that represents the inactivation characteristics of a BI. For a moist heat process, the process variables are time and temperature in the presence of moisture (e.g., 132°C for 4 minutes in saturated steam). ISO 11140-1 also specifies how both types of CI should react when tested in pass and fail conditions.

Type 4 CIs should reach their endpoint when exposed to their stated values declared by the manufacturer. However, a Type 4 CI need not react to all the process variables required to achieve sterilization. Similarly, the SVs for the process variables to which the Type 4 CI reacts need not relate to an acceptable time and temperature of exposure required for sterilization. Thus, for example, a Type 4 might respond to temperature and time with no mention of the impact of moisture on the indicated result. The declared SV for temperature could be 130°C, and that for time, 2 minutes. A CI with such SVs would be of little use when monitoring a sterilization process that must attain at least 132°C and expose the load for at least 4 minutes.

The requirements for Type 5 CIs in ISO 11140-1 are far more prescriptive, requiring a minimum level of performance that is linked to the inactivation of a BI meeting ISO 11138. ISO 11140-1 requires that the manufacturer of a Type 5 CI must declare SVs at 121°C and 135°C, and these must be greater than 16.5 and 1.2 minutes, respectively. The CI manufacturer must also declare a third SV, which should fall between 121 and 135°C, ideally close to the midpoint. Having three SVs provides the user with a clear indication of how the CI will react under different processing conditions.

When tested according to ISO 11140-1, both Type 4 and 5 CIs must show an endpoint (a “pass” result) when exposed to their stated values. Similarly, ISO 11140-1 also specifies test conditions where the CI should not reach its endpoint (a “fail” result). Figure 2 is an example of these requirements, shown graphically, in which the CI has declared SVs of 132°C and 4 minutes. When tested under these conditions, the CI should show a “pass” result. When the SV for temperature is reduced by 2 (to 130°C) and the SV for time by 25% (to 3 mins), the CI must show a “fail”. Close examination of the figure shows several grey areas where performance is not defined, such as a higher temperature for a shorter time or a lower temperature for a longer time.

Figure 2. Test conditions specified in ISO 11140-1 for a Type 4 CI exposed to its stated values of 132°C and 4 minutes, which define a “pass” condition and values under which a “fail” should be indicated. Note the grey areas where performance is not defined.

Examples of the requirements for a Type 5 CI are shown in Figure 3 (plotted as log₁₀ of exposure time versus temperature on a linear scale) and Table 3. ISO 11140-1 specifies SVs at 121 and 135°C, which should be greater than 16.5 and 1.2 minutes, respectively. The manufacturer must also declare a third SV within the temperature range, preferably near the midpoint, which in this example is 4.4 minutes at 128°C. When tested under these conditions, the CI should show a “pass” result. When Type 5 CIs are tested under “fail” conditions, the SVs for exposure temperature are reduced by 1°C (i.e., to 120 and 134°C), and the third SV-1°C (e.g., 127°C). Simultaneously, the SVs for time are reduced by 15%. Under these conditions, the CI must show a “fail” result.

Figure 3. Test conditions specified in ISO 11140-1 for a Type 5 CI exposed to the specified stated values at 121°C, 135°C, and the manufacturer's third declared value (in this example, 128°C), all of which define a “pass” condition (green zone). When the temperature is reduced by 1°C and time by 15%, a “fail” result should be observed (red zone).

Table 3. Test exposure conditions for a “pass” or “fail” result for Type 4 or Type 5 CIs.

Using Chemical Indicators

Various guidance documents specify that CIs should be placed in every sterile pack. The United States Association for the Advancement of Medical Instrumentation (AAMI) guidance document ST79⁷ requires the use of a CI inside each package of goods sterilized. Other national guidance documents specify similarly. In an effort to reduce the number of mistakes during surgical intervention, the WHO created the Surgical Safety Checklist (SSC),¹⁰ which theatre teams are encouraged to adopt and has been shown to improve patient outcomes.¹¹ The SSC has several simple questions that should be answered before, during, and after a surgical procedure. One of these questions is, “Has sterility (including indicator results) been confirmed?” This can be interpreted as a question of whether the external CI shows that a load item has been processed. However, an outside CI provides no evidence of satisfactory steam penetration through the sterile barrier system into the pack. For this reason, an internal CI should also be present and checked to establish that sterilizing conditions were met at the point of placement inside the pack. This is the purpose of the internal CI, which, if placed in the most difficult to penetrate part of the pack, will provide strong evidence that satisfactory processing has taken place.

Choosing a Chemical Indicator

The choice of the type of CI to use is very important. The outside of each pack should have a Type 1 process indicator present to provide immediate evidence that a pack has been through a process and not simply transported unprocessed.¹² The choice of an internal CI is more complex. Several questions should be considered when choosing a CI:

1. Does the CI react to all the process variables for a steam sterilization process (i.e., time, temperature, and the presence of moisture)?
2. Are the CIs stated values related to the steam sterilization process in use (i.e., the sterilization temperature and the exposure time in saturated steam)?
3. Does the CI meet its specified claims for SVs? Data should be available from the manufacturer.
4. Is the CI color change easy to read? If the starting color is only slightly different from the endpoint color, then users will find it difficult to interpret the result.
5. What is the variability associated with the attainment of the color change? If the color change reaction is highly variable, then for any given exposure condition, some CIs might show a “pass” result and others a “fail”.

Table 4 provides a comparison of the attributes of Type 5 and Type 4 CIs, answering the questions posed above.

Table 4. A comparison of the attributes of Type 5 and Type 4 CIs.

Conclusions

When choosing a CI for placement inside sterile instrument packs, the practitioner is faced with a multitude of choices. The practitioner should consider the attributes of Type 4 and 5 CIs when making such choices and consider carefully the declared performance characteristics required by ISO 11140-1, the readability (color change), and the variability of the response of the CIs on offer.

References

1. McDonald, G and Hansen, J. Block’s Disinfection, Sterilization and Preservation, 6^th Ed, 2021, Wolters Kluwer, USA
2. ISO 14937:2009, Sterilization of health care products – General requirements for characterization of a sterilizing agent and the development, validation and routine control of a sterilization process for medical devices (ISO 14937:2009), International Standards Organisation, Geneva, Switzerland.
3. ISO 17665:2024, Sterilization of health care products – Moist Heat – Requirements for the development, validation and routine control of a sterilization process for medical devices (ISO 17665:2024), International Standards Organisation, Geneva, Switzerland.
4. ISO 11138-3:2018, Sterilization of health care products – Biological indicators – Part 3: Biological indicators for moist heat sterilization processes (ISO 11138-3:2017), International Standards Organisation, Geneva, Switzerland.
5. ANSI/AAMI/ISO 11140-1:2014, Sterilization of health care products – Chemical indicators – part 1: General requirements. AAMI, N Glebe Rd, Arlington, VA
6. Bowie, J. et al, The Bowie and Dick autoclave tape test, Lancet (16), 586-587 (1963)
7. ANSI/AAMI ST79:2017, Comprehensive Guide to steam sterilization and sterility assurance in health care facilities, 2021, AAMI, N Glebe Rod, Arlington VA
8. Perkins, J,J. Principles and Methods of Sterilization, 1^st Ed, 1956, Charles Thomas Publishers, Springfield Il, USA.
9. Bunn, J.L and Sykes, I,K. A chemical indicator for the rapid measurement of Fo values, J Appl Bacteriol, 51, 143-147, 1981
10. World Health Organisation (WHO), Surgical Safety Checklist 2009, http://whqlibdoc.who.int/publications/2009/9789241598590_eng_Checklist.pdf?ua=1
11. Bergs, J, et al, Br J Surgery, 101(3), 2014, Meta analysis of published data examining the impact of WHO SSC compliance.
12. Lagoe, A.J, Whistleblower: Non sterile equipment used on patients https://eu.usatoday.com/story/news/investigations/2014/09/18/whistleblower-non-sterile-equipment-used-on-patients/15827577/