Steam Sterilization Cycles in Healthcare Facilities: What to Use and When to Use It

December 1, 2004

Steam Sterilization Cycles in Healthcare Facilities: What to Use and When to Use It
Biological Indicators for Monitoring

BySandra Lee, BSM, RN

What is a biological indicator?

According to the Association for the Advancement of MedicalInstrumentation (AAMI) document Steam Sterilization & Sterility Assurancein Health Care Facilities, ST46, a biological indicator is an inoculatedcarrier contained within its primary pack, ready for use, and providing adefined resistance to the specified sterilization process.

Food and Drug Administration (FDA) regulations state, abiological sterilization process indicator is a device intended for use by ahealthcare provider to accompany products being sterilized through asterilization procedure and to monitor adequacy of sterilization. The deviceconsists of a known number of microorganisms, of known resistance to the mode ofsterilization, in or on a carrier and enclosed in a protective package.Subsequent growth or failure of the microorganisms to grow under suitableconditions indicates the adequacy of sterilization. [21 CFR 880 2800 (a) (1)]

Does a negative biological test result prove thateverything in the load is sterile?

It is important to understand that a biological indicator isused to determine if the set sterilization cycle parameters were sufficient tokill the test microorganisms. Remember that a negative biological result does not prove that everything in the load is sterile nor doesit prove that all items were adequately exposed to sterilization conditions. Theprobability of sterilization of an item(s) is not only dependent on a properlyfunctioning sterilization cycle, but also on properly decontaminated, assembled,wrapped, and loaded items that can all be adequately exposed to sterilizationconditions.

Which test microorganism is used to monitor steamsterilization cycles?

Spores of Geobacillusstearothermophilus are used to comply with theAAMI/American National Standards Institute (ANSI) standard titled Sterilizationof Health Care Products-Biological Indicators-Part 3: Biological Indicators forMoist Heat Sterilization, ST19. Geobacillusstearothermophilus is not harmful to humans. Thisspore/microorganism is incorporated into a process monitoring device to teststeam sterilization cycles.

What are process monitoring devices and how are theyconfigured?

There are currently two types of process monitoring devicesavailable for use in healthcare facilities. They are also known as biologicalindicators or process challenge devices (PCDs). According to the AAMI, A PCDis an item used to assess the effective performance of a sterilization process,which is designed to simulate product to be sterilized and which constitutes adefined challenge to the sterilization process. The conventional biologicalindicator is a carrier (usually a spore strip/disc) that is inoculated with Geobacillusstearothermophilus, placed into its primary pack,which is a self-contained unit that also houses a unit of growth medium, and isready for use. This self-contained unit is used by itself to test flashsterilization cycles, and is contained in a prescribed AAMI test pack orequivalent commercially prepared test pack to monitor steam sterilization cyclesfor wrapped goods. Death of the Geobacillusstearothermophilus indicates that appropriateparameters for sterilization were achieved. A biological indicator withenzyme-based early-readout capability is also available. Its construction isessentially the same as the conventional biological indicator, but anon-fluorescent substrate is incorporated into the medium. After sterilizationand a rapid incubation time, a special reader is used to detect fluorescencethat results from an enzymatic breakdown of the non-fluorescent substrate. Thisfluorescence indicates an ineffective steam sterilization process. If nofluorescence occurs, the enzyme has been inactivated, which predicts that thebiological spores will not grow upon further incubation.

Is a rapid enzymatic indicator a biological indicator?

A rapid enzymatic indicator contains enzymes that have beenisolated from spore-forming bacteria. It is not a biological indicator becauseit does not contain bacterial spores. The enzymes have a temperatureand pH sensitivity that is similar to the microorganism they came from. When theenzyme is exposed to steam sterilization conditions, its activity is lost overtime and is said to correlate to the destruction of Geobacillusstearothermophilus. Incubation is not required becausethere is no microorganism included in this test material, but an activationsolution is applied to the sterilized indicator vial and the color changeindicates a positive or negative result.

When is it necessary to biologically monitor steamsterilization cycles?

According to AAMI (ST46), conventional biological indicatorsshould be used to monitor steam sterilization cycles under the followingconditions:

  • Upon installation of a new sterilizer

  • After relocation of an existing sterilizer

  • After a sterilizer malfunction

  • After a sterilizer process failure that is indicated by a positive biological test

  • After major repairs to a sterilizer that are outside the scope of routine or preventive maintenance (Examples of major repairs would be: weld repairs to the sterilizer chamber/pressure vessel; repair of the chamber door; repair of a major plumbing assembly; or rebuilds or upgrades to the sterilizer controls)

  • After repairs to the steam generator/delivery system

  • When performing periodic quality assurance testingof items that you routinely sterilize The use of biological enzyme-based early-readout or rapid enzyme indicators is not acceptable under thecircumstances listed above because the best way to verify the lethality of asterilization cycle is to kill large populations of microorganisms that areresistant to steam sterilization.

Routine biological monitoring of steam sterilizer cycles(gravity, pre-vacuum, and flash cycles) is also the standard, as well asadditional biological monitoring when implantable medical devices are in a load. In this case, either conventional biological indicators orenzyme-based early-readout indicators can be used.

Which process monitoring device should I use to performroutine biological monitoring of steam sterilization cycles and loads withimplantable medical devices? How often do I need to do it?

Conventional biological indicators and enzyme-basedearly-readout indicators can each be used to routinely monitor steamsterilization cycles and serve as the basis for release of sterilized items foruse, including implantable medical devices. Routine monitoring is performed in asterilization load at least weekly, but preferably every day the sterilizer isin use (testing every sterilizer and type of sterilization cycle used). Everysterilization load that contains implantable medical devices must bebiologically monitored. For maximum sterility assurance, implantable medicaldevices should be quarantined until negative biological test results areconfirmed.

If I use an enzyme-based early readout indicator, do I need todo anything else to verify the performance of a steam sterilization cycle?

Yes. According to AAMI (ST46), the use of this method ofmonitoring requires that the performance of the sterilization cycle beperiodically verified by one of two methods. Use one of the following testmethods at least weekly, but preferably every day that the sterilizer is used:

  • Continue incubation of the enzyme-based early-readoutfollowing the manufacturers instructions. The amount of time necessary to ensure that any survivingmicroorganisms will grow out will be specified by that manufacturer.

  • Use a conventional biological test pack to verifysterilizer performance.

Details regarding monitoring steam sterilization cycles can befound in AAMI ST46, and ST47, Flash Sterilization: Steam Sterilization of Patient Care Items for Immediate Use.

Sandra A. Lee,BSM, RN, is the senior manager of professional education for STERIS Corporation.Her professional experience includes medical-surgical and perioperative nursing,central service management, materials management, and project management/systemplanning. Since 1982 Lee has actively participated in the writing and continuousupdating of decontamination and sterilization standards and recommendedpractices with the AAMI, and she is a member of the AAMI Sterilization StandardsCommittee. She is also a member of the Technical Committee on Sterilization atCSA International in Canada. In addition, she serves as a U.S. representative tothe European Committee for Standardization (CEN) as an ISO Technical AdvisoryGroup 198 liaison.