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spore, by reading a fluorescent product that is produced when this enzyme con- verts the non-fluorescent substrate in the media vial.4


The fluorescence indicates the


presence of an active enzyme and a steril- ization process failure. Non-fluorescence indicates inactivation of the enzyme and an effective sterilization process. A ster- ilization process failure can be detected in as little as a few minutes, which is an improvement over the one to seven days previously required.5


Obtaining results


within a minimal incubation time allows sterilization process failures to be iden- tified much sooner, instruments to be turned around faster, costs associated with inventory and recall to be reduced, and improved patient outcomes.6


When a biological indicator is doing its job


BIs detect conditions that are not able to kill the spores. Since spores are more resistant than other microbes, they pro- vide a safety margin. If the spores have been killed, then by inference, the other microbes on medical devices should have also been killed. Sterilization cycles are designed to kill spores within the first half of the exposure cycle. In a normally functioning cycle, the spores should easily be destroyed. See Figure 2, below, for a graphic representation of spore kill in a typical sterilization process. At the beginning of the process, all spores are expected to be alive. By the middle of the process all spores should be killed. At some point between these two states, marginal cycle conditions exist. A marginal cycle is one that fails to completely kill all spores, and can yield both positive and negative BI results. In a sterilization process failure (i.e., steril- izer not functioning, inadequate steam quality and quantity, or human errors due to incorrect packaging, loading, or choosing the incorrect cycle for the load) the marginal part of the process may


come at the end of the cycle. It is toward the end of these marginal cycle condi- tions where a more sensitive indicator, such as a fluorescent dye, may detect a few more positives than a less-sensitive indicator, such as a pH dye. Detection of biologically active proteins, such as the intrinsic enzyme in the spore that breaks down the glucoside substrate containing the fluorescent dye, demonstrates a steril- ization process failure. Whether the spore is able to multiply or not, the detection of biologically active proteins demonstrates a sterilization failure.


Rapid readout BIs can detect marginal cycle conditions that other spore strips and self-contained BIs do not. Vesley and Allwood concluded in their evaluation of BIs that rapid readout BI technology was a more sensitive indicator of marginal ster- ilization cycles than other self-contained BIs without any indication of false positive results.7


Likewise, Rutala et al. reported


that rapid readout BIs were a suitable monitor that ensures sterilization with- out inappropriately indicating failure.8


Recommended practices for using biological indicators A process challenge device (PCD) is a test pack that creates a challenge to the sterilization process that is greater than or equal to a routinely processed item.9 The AAMI and the Association for the periOperative Registered Nurses (AORN) recommend that a BI in a PCD be run weekly, but preferably every day that the steam sterilizer is used.1,10


Additionally BIs


within PCDs should be used for sterilizer testing after sterilizer installation, reloca- tion, malfunction or process failure, and any major repairs.1,9


This testing should


be done in each type of cycle (gravity- displacement, dynamic air-removal [pre- vaccum or steam flush pressure pulse]) used. If a sterilizer runs cycles for different exposure times, then the shortest cycle time should be tested.1


In addition, when


using the immediate-use-steam-steriliza- tion (IUSS) sterilization process, each type of tray configuration (e.g., open surgical tray, single-wrapped surgical tray, protec- tive organizing case, rigid sterilization container) in routine use should be tested separately. Each load containing implant- able medical devices should be monitored with a PCD containing a BI and a Type 5 integrating chemical indicator, and the implantable device quarantined until the results of the BI testing are available. If a PCD containing only a BI is used to release a sterilized load, it should be quarantined until the BI results are known.1 BIs are used for qualification testing by the sterilizer manufacturer at time of installation, and by the healthcare facility for periodic quality assurance testing. BIs are also used for product testing.1


Summary


Biological indicators (BIs) provide direct evidence that the sterilization process con- ditions are able to kill spores. BIs have evolved over the past 50 years. Results that once took seven days or more now are obtained in less than 1 hour, and less than 30 minutes in some cases. Cumbersome subculturing and long incubation times have been replaced by self-contained biological indicators with rapid readout techniques. HPN


References


1. Association for the Advancement of Medical Instrumenta- tion. Comprehensive guide to steam sterilization and sterility assurance in healthcare facilities. ANSI/AAMI ST79: 2017.


2. Association for the Advancement of Medical Instrumen- tation. Sterilization of health care products – Biological indicators – Part 2: Biological indicators for ethylene oxide sterilization processes. ANSI/AAMI/ISO 11138-02:2017.


3. Association for the Advancement of Medical Instrumenta- tion. Sterilization of health care products – Biological indica- tors – Part 3: Biological indicators for moist heat sterilization processes. ANSI/AAMI/ISO 11138-03:2017.


4. Schneider PM. Evaluation of a new rapid readout biological indicator for use in 132°C and 135°C vacuum-assisted steam sterilization cycles. Am J Infect Control. 2014 Feb;42(2):e17-21.


5. Alfa MJ, Olson N, DeGagne P, Jackson M. Evaluation of rapid readout biological indicators for 132 degrees C gravity and 132 degrees C vacuum-assisted steam sterilization cycles using a new automated fluorescent reader. Infect Control Hosp Epidemiol. 2002 Jul;23(7):388-92.


6. Zaman SU, Sadia I, Yasmin N, et al. Application of Rapid Biological Indicators Coupled with Auto-Reader for the Quality Assurance of Surgical Instruments after Sterilization at a Car- diac Hospital in Bangladesh. Cureus. 2021 Nov 10;13(11):e19428.


7. Vesley D, Nellis MA, Allwood PB. Evaluation of a Rapid Readout Biological Indicator for 121ºC Gravity and 132ºC Vacuum-Assisted Steam Sterilization Cycles. Infection Control and Hospital Epidemiology, Vol. 16, No 5, July:1995.


8. Rutala WA, Jones SM, Weber DJ. Comparison of a rapid readout biological indicator for steam sterilization with four conventional biological indicators and five chemical indicators. Infect Control Hosp Epidemiol. 1996 Jul;17(7):423-8.


9. Association for the Advancement of Medical Instrumenta- tion. Process Challenge Devices/Test Packs for Use in Health Care Facilities. AAMI TIR31:2008.


Figure 2. Spore kill in a sterilization process. 42 June 2023 • HEALTHCARE PURCHASING NEWS • hpnonline.com


10. Association of periOperative Registered Nurses. Guideline for Sterilization. In: Guidelines for Perioperative Practice. 2023.


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