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Chemical Indicators: What to Use, When to Use It, and What’s the Difference between Them All?

By Jonathan A. Wilder, Ph.D., Managing Director

Chemical sterilization indicators (CIs) are vital tools in an SPD, but since the last revision of AAMI/ANSI/ISO 11140-1, there are so many types of chemical indicators. Classes are dead. Long live the types! So what do you do with them?

As always, AAMI ST79 is the definitive reference, but take it with a grain of salt because you also have to follow the indicator manufacturers’ instructions for use. To ensure we are all speaking the same language, descriptions of each type of indicator are in a glossary at the end of this article.

Let’s begin with chemical indicators. But first, does a chemical indicator tell you that the load is sterile? No, no, no! Only a biological indicator (BI) can tell you that, and only at the location where it was in the load. The rest of the load could be carrying a plague, but you really can’t say that the BI tells you that the rest of the load is sterile.

Now that I’ve scared you, the reality is that a monitoring program using BIs, CIs, and review of the sterilizer cycle logs will give you excellent assurance that the load has been sterilized if you have followed the manufacturers’ instructions for use for the instruments, the sterilizer, and the indicators.

Is the sterilizer working right?
Before anything else, like anything else, you have to begin in the morning. For steam sterilizers, what do you do? Run a Bowie-Dick test. Technically, these are Type 2 indicators. They tell you that the sterilizer is doing an adequate job of removing air from the chamber and that there aren’t any major air leaks. This sort of testing only applies to prevacuum steam sterilizers.

Other sterilization technologies do not have a similar test type, but they do have requirements for routine monitoring, which we will discuss later.

Has it been processed?
You want to know at a glance if an item has been processed. For this, you use a Type 1 chemical indicator. What are examples of a Type 1 chemical indicator? Tape. Dots. Pouch indicator spots. Labels. Container locks with dots on them. They tell you that the item has been processed, but not that it is sterile. They aren’t made to do that. Where do these go? On the outside of the package or container so you can see them at a glance.

Integration, the whole story
The U.S. Food and Drug Administration (FDA) has a catch-all chemical indicator clearance for integrators. What’s an integrator? It is a chemical indicator that integrates the critical parameters of the process to tell you if the product is likely to be sterile. These exist for steam and ethylene oxide (ETO) processes, but not for peroxide, plasma, and ozone. Why not? There is no standard for them yet and the technology may not be there, but that’s a topic for another article.

An integrator typically responds to all critical parameters of a sterilization process. For steam, these are time, temperature, and the presence of moist, not dry heat. For ethylene oxide, these are humidity, ethylene oxide concentration, and time. Some integrators’ (Type 5) performance is designed to mimic a biological indicator’s performance. Some are designed to respond to the actual parameters of the process (Type 6). Some cover most of these (Type 4). The difference between Type 5 and Type 6 indicators is that a Type 6 indicator is designed to show a passing result only when the process is mostly complete. Type 5 indicators, such as the biological indicators whose response they mimic, have performance requirements based on exposure to half of the lethal sterilization exposure time. Both give you an excellent margin of safety.

Where do I put them? Everywhere. Seriously, put them where the process might have some problems to achieve sterility, such as inside each package where they will be seen when opened in the sterile field. For multilayer trays, use at least one per layer, preferably more, with special attention given to areas where a heavy item is located. Place one between an item and the part of the tray or container holding it. In other words, place them in the easy-to-sterilize locations and the hard ones, too.

What’s a PCD?
The acronym PCD stands for process challenge device. It takes a normal indicator, usually a BI with a CI integrator or emulator included, and a Type 1 process indicator on the outside of the PCD. These are designed to challenge the process to a greater degree than an indicator alone. A PCD has to provide equal resistance to test packs defined in the standards that apply to the sterilization method and indicator type.

The direction of this article has been toward steam sterilization, which is the most prevalent. For other sterilization methods, there are also manufacturers’ and standards’ requirements for routine (daily) testing. Ethylene oxide, vaporized hydrogen peroxide (VHP), vaporized hydrogen peroxide/plasma, and vaporized hydrogen peroxide/ozone manufacturers all recommend at least daily monitoring of the cycle, preferably with each cycle run. Each manufacturer will tell you where to put the indicator or indicator system in the chamber or load.

Not that you have a bit more information, it’s up to you to use it. And I know you can!

Glossary: Chemical Indicator Types
Type 1: The process indicator. These are typically found on tape for wraps, self-adhesive dots, printed on the outside of a pouch, on container locks, pretty much anywhere you want a quick indicator that the item was processed. But I said processed, not sterilized. These indicators tell you that the item was in a sterilizer, and the process ran, but they tell you nothing about the sterility of the item. For that you need to move up the food chain.

Type 2: The Bowie-Dick test. This is not a sterilization indicator. This is used exclusively in prevacuum steam sterilizers to verify that the sterilizer’s prevacuum conditioning phases effectively remove air from the load. Again, like the Type 1 indicator, these indicators tell you nothing about sterility. Just that the vacuum system is working right and there are no major air leaks.

Type 3: These are single-parameter indicators (temperature, sterilant present, humidity) and, to my knowledge, are not sold in the healthcare market.

Type 4: Two-parameter indicators. These can be cleared by the FDA as integrators, as can Type 5 indicators. The two parameters here are normally time and temperature. These indicators can show you how close you are to success or failure in a cycle.

Type 5: Integrating indicators. These give you the whole story about the process. It is required to react to all critical process variables. For steam, the variables are time, temperature, and the presence of moist heat (not just dry heat). For ethylene oxide (ETO), the variables are gas concentration, temperature, humidity, and exposure time. These do not yet exist for VHP/plasma processes because there is no standard for them. These indicators’ response to the process is similar to that of a biological indicator, but without biological species, of course.

Type 6: Emulating indicators. These indicators are process-specific, for certain temperatures and times, like 273°F (134°C) and 3:30 minutes. They are designed to respond with a Pass result only when the cycle is correctly run for a goodly portion of the cycle. More recently, there have been indicators with ranges of response. Note that the FDA clears these as integrators, so any additional information given by the Type 6 indicator is for your information only. It does not provide a better result when you are audited, or it shouldn’t.

For more great information about CIs and BIs, check out the second annual Beyond Clean LIVE panel discussion on the topic of sterility assurance.

Dr. Jonathan Wilder has worked with all thermal and chemical sterilization methods, as well as cleaning and disinfection methodologies, bringing his background in physical chemistry and surface physics to bear upon difficult problems in the field. He has been an active participant in U.S. and international standards development since 1998 through AAMI. As of January 2018, he is the cochair of the U.S. standards-making committee for hospital steam sterilizers.

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