A Closer Look at ST91:2021 for Endoscope Processing


By: Susan Klacik, Clinical Educator, HSPA

March 3, 2022

Categories: AAMI News, Sterilization Professionals

This in-depth review of ANSI/AAMI ST91:2021 by the Healthcare Sterile Processing Association is also found at myhspa.org.

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On March 3, 2022, AAMI released the updated ANSI/AAMI ST91:2021 Flexible and semi-rigid endoscope processing in health care facilities. The revised version reflects current research and advancements in flexible endoscope processing. What follows is an abbreviated summary of some of the key revisions and additions.

Research has shown there have been multi-drug-resistant organism (MDRO) infection outbreaks related to flexible endoscopes. Review of the processes and procedures used to reprocess these complex and sophisticated devices revealed failures in reprocessing steps that could jeopardize patient safety. In fact, multiple peer-reviewed publications in several countries, including the U.S., have documented breaches in processing that led to patient exposure to improperly processed flexible and semi-rigid endoscopes, and subsequent infections. Mitigating these risks hinges on proper care and handling—from point of use through all reprocessing steps [including complete drying after high-level disinfection (HLD)] and then to safe transport of the devices for patient use.

Proper cleaning of flexible endoscopes is critically important. The revised ST91 includes additional recommendations for decontamination sinks. Three sinks are ideal, according to the standard (one sink used for leak testing, another for manual cleaning and the third for critical rinsing); however, at this time, two sinks or one sink with two separate basins may be used. There should also be additional space provided for delayed processing. Sink size and location(s), flushing accessories and ergonomics are all important considerations are to facilitate effective cleaning as well as employee safety. Sink size can affect processing because tightly coiling of the endoscope could damage components, including image or light bundles, internal channels, tubes, and/or angulation wires. Using ergonomic height-adjustable sinks helps personnel avoid bending over to perform cleaning tasks. Sinks located too high or too low increase the risk of back injury or strain.

The updated ST91 is available at www.aami.org/st91; those with an electronic subscription will have immediate access to the revised standard. The standard is also available to HSPA members at a reduced rate (www.iahcsmm.orgunder the “education” tab in the publications section).

Training, competency, certification recommendations

It is recommended that all personnel performing endoscope processing duties complete formal training and competency verification in all aspects of endoscope processing prior to their first solo assignment to perform these tasks. Additionally, the standard recommends that all personnel performing processing of endoscopes should be certified in flexible endoscope processing within two years of employment and should maintain that certification throughout their employment.

To provide a higher level of assurance, evidence supports the sterilization of all flexible endoscopes, including those used in both semi-critical and critical procedures. Factors supporting this transition include high microbial load after patient procedure, the complex design of flexible endoscopes, and the risk for biofilm formation on these devices. Sterilization can reduce this risk because of the greater margin of safety in the overkill process, and it provides a sterile packaged endoscope. HLD is a multi-step process that is expected to inactivate most pathogenic bacteria, viruses and fungi; however, it may not reliably inactivate certain types of microorganisms, including bacterial spores. The ST91 standard committee recognizes that transitioning from HLD to sterilization as the standard of care will take time in terms of the endoscope and sterilizer manufacturers implementing necessary technological advances and for healthcare facilities to provide the budgetary and site accommodations to implement this change. The standard does not mandate sterilization, but recommends healthcare facilities begin making the necessary steps toward sterilization for flexible endoscopes, when possible.

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Considerations for environment, HVAC monitoring, water quality 

Just as the environmental requirements were changed in the ANSI/AAMI ST79:2017  Comprehensive guide to steam sterilization and sterility assurance in health care facilities, they were also changed in ST91. It is recommended that heating, ventilation and air conditioning (HVAC) operating parameters in the endoscope processing area should comply with the specifications of ANSI/ASHRAE/ASHE 170 that were in effect when the HVAC system was initially installed or last upgraded. The standard also recommends that healthcare facilities  establish and implement processes for monitoring HVAC performance parameters and have a mechanism in place for identifying and resolving variances within the rooms throughout the facility where processing occurs. Portable fans are not permitted in the processing area, other than exhaust fans on ventilation systems and installed and operated fume control hoods. The facility engineering personnel (or designated responsible personnel) should establish policies and procedures for monitoring and maintaining HVAC parameters within the processing areas. These procedures should include maintaining records of monitoring results that are retrievable either from a central system or a local log. If a variance in the HVAC parameters occurs, processing personnel, in combination with a multidisciplinary team such as representatives from facility engineering, infection prevention, risk management, endoscopy, perioperative, sterile processing leadership or other designated personnel, should conduct a risk assessment. The processing department is defined by ANSI/ASHRAE/ASHE 170 as a critical area.
What are the key areas of the document that IPs can focus on to ensure their facility is compliant with the new standard? Don’t miss:ST91:2021 Webinar

Evidence has shown the importance of appropriate water quality across all stages of endoscope reprocessing processing. For that reason, a new section has been added to ST91 (Section 4.3.11 Water quality). To ensure that the correct water quality is used in each stage of processing, the manufacturers’ written instructions for use (IFU) for all equipment and supplies should be used.  This includes endoscope and accessories, automatic endoscope reprocessors (AERs) and processing chemicals used. Quality water helps prolong the life of medical devices, facilitates effective functioning and reduces the risk of medical device contamination. The healthcare facility should monitor and control the water supply quality to endoscope processing sinks and processing equipment. Whenever major repairs occur or changes to the water utility system are made, equipment qualification testing should be performed before use. Note: Additional information on water quality can be found in AAMI TIR34 Water for the reprocessing of medical devices.

Maintaining a clean environment for endoscope processing  reduces the risk of transmission of  microorganisms. It is recommended that a multidisciplinary team be established to select cleaning chemicals, materials, tools and equipment for use in the endoscope processing setting. Cleaning frequencies should be established for high-touch objects and surfaces, as well as for storage shelves, endoscope storage cabinets, and similar objects. Terminal cleaning frequency should also be established for endoscope processing areas (at a minimum, daily, and more often as needed). Environmental cleaning should be verified using methods such as ultraviolet light visible markers, protein tests and ATP bioluminescence to measure the adequacy of environmental cleaning.

Precleaning at the point of use has been modified to include all aspects of point-of-use treatment. Point-of-use treatment now includes hand-off communication from point of use to the decontamination area (to include, at minimum, patient identifier, date of procedure, time point-of-use treatment was completed, and employee contact).

Expanded transport section, revisions to leak testing recommendations

The transport section regarding used endoscopes has been expanded. One recommendation involves keeping the endoscope and accessories moist by applying a detergent designed to retain moisture, placing a towel moistened with water over the items or placing items inside a package designed to maintain humid conditions. Keeping the endoscopes and accessories moist prevents organic material from drying on the surfaces and makes cleaning of these devices and accessories easier. Other steps in the transport of used endoscopes have been expanded to include hand-off communication from the transporter to the processing location (e.g., confirmation that the endoscope was transported correctly, and all accessory parts are present; the date and time of completion of use of that device;  the time of point-of-use treatment; and the patient identifier).  

Significant revisions were made regarding leak testing, including having automated leak testers placed on a calibration schedule to verify the leak tester is producing the correct pressure. It is possible for leak testers to fail, and manual handheld leak testers and leak tester tubing should be inspected for damage, leakage and pressure output. Pressure verification should be performed for each type of leak tester in the facility each day endoscopes are used; calibration ensures that an endoscope will be challenged with the proper pressure each time when being tested for leaks. Documentation of the leak testing results should be recorded.

Some AERs include mechanical leak testing, and the updated ST91 recommends that when using this type of AER, the endoscope and AER manufacturers' written IFU must be followed (with the outcome of the leak test documented). Note: Conducting mechanical leak testing using an AER cannot substitute for the leak testing recommended in the endoscope manufacturer’s IFU.

Additional guidance was added for leak test failures, including labeling the endoscope to identify it as defective equipment, removing it from service, and following facility guidelines for repair. The failure should be reported according to the organizational policy and procedure, including the endoscope product identification and traceability information.

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Cleaning and inspection steps clarified, broadened

Significant changes were made to the cleaning section of ST91. The first change relates to the timing of the cleaning process after use. Typically, the IFU contains this type of information. If a delay in endoscope processing the endoscope exceeds the manufacturer’s recommended time, the endoscope should be cleaned following the  manufacturer’s written IFU for delayed processing. Section 7.6 of ST91 provides the typical cleaning flow for endoscopes, their removable parts, and endoscopic accessories; however, validated instructions may vary. It is important to ensure that cleaning is initiated in the timeframe prescribed in the manufacturer’s written IFU. If no timeframe is given, manual cleaning should be initiated within one hour or as determined by the facility based on their own documented risk assessment.

The previous version of ST91 recommended keeping all reusable removable parts, such as the valves and caps, together. This practice has been modified in the updated version. Detachable parts and valves pose unique concerns. To minimize risk of transmission of infection, any one of the following measures are now being recommended:

1. Use of single-use biopsy port caps and single-use valves

2. Sterilization of reusable valves and caps, preferably using steam sterilization when materials are compatible.

3. HLD of valves and caps (either keeping the valve/caps together with the endoscope as a set or tracking the individual valve/cap to the patient and the procedure).

ST91 now also recommends cleaning and disinfecting sinks between uses. Significant changes were made to Section 7.8 Inspection and cleaning verification. Recent outbreak investigations have shown that visual inspection of endoscopes can identify defects or residual contamination. An endoscope that appears clean can harbor debris that cannot be seen without magnification. Cleaning verification used after endoscope cleaning and before disinfection or sterilization may detect residual organic soil and microbial contamination present on a surface, even if the devices appear clean.  The use of cleaning verification was included in the previous flexible endoscope standard; however, it did not specify how often to perform the test. The updated version recommends cleaning verification tests be performed after each use of  high-risk endoscopes, notably duodenoscopes, linear ultrasound (EUS) endoscopes, bronchoscopes, endobronchial ultrasound (EBUS) endoscopes, ureteroscopes, cystoscopes, and others as determined by the facility.

The use of borescopes is now recommended in the revised ST91. Inspection using a borescope allows for visualization of the internal working channels to identify unacceptable damage. It is important to check the endoscope manufacturer’s written IFU, maintenance bulletins, and other labeling for direction on what is considered a defective or damage device. The updated standard does not specify how often borescope inspection should occur and recommends that the frequency be determined by the facility. If a borescope is used as a quality monitor after disinfection/sterilization,  the endoscope will need to be processed again before clinical use. As with all medical devices, the borescope should be processed in accordance with the manufacturer’s written IFU.

Heightened focus on endoscope drying

Thorough drying has been shown to reduce the risk of microbial contamination following high-level disinfection from the recontamination of the endoscope by waterborne microorganisms during rinsing. The presence of these  microorganisms in conjunction with retained moisture can lead to the development of biofilm and increase patient infection risks. Recent research has led to many revisions to ST91’s drying recommendations after completion of the cleaning and disinfection process. Based on evidence, it is recommended that endoscope channels be dried for a minimum of 10 minutes with pressure-regulated forced instrument air (or, at minimum, HEPA-filtered air). If  moisture is still observed, the drying time should be extended until no moisture is visible. The endoscope manufacturer’s written IFU should be reviewed for the maximum PSI. Note: Manual drying should occur even if an AER is used and it has an air purge or extended dry time feature.

The use of alcohol has long been recommended in the drying process after disinfection is completed; however, some studies have shown that alcohol can be a fixative agent. For that reason, ST91 now recommends that a multidisciplinary team (with representation from infection preventionists, endoscopy and perioperative nurses, endoscope processing personnel, endoscopists, and other involved personnel) conduct a risk assessment to determine whether endoscope lumens should be flushed with 70% to 90% ethyl or isopropyl alcohol.

Additional language was added to the ST91 section, Terminal sterilization by gaseous chemical sterilization processes. Sterilization of endoscopes is recommended because sterilized endoscopes provide an increased safety margin over HLD-processed endoscopes. Terminally sterilized endoscopes are completely dry and packaged (thereby reducing the chance of contamination for longer periods of time), and patient-ready. Packaged endoscopes have tamper-evident seals that clearly distinguish used endoscopes from “patient-ready” endoscopes.

New recommendations were also added regarding the storage of liquid chemically sterilized endoscopes. Liquid chemically sterilized endoscopes intended for use as critical devices should not be stored with a claim as being patient-ready; instead, they should be used immediately after processing. They may also be used immediately in semi-critical applications. If endoscopes are processed using a liquid chemically sterilization method but are not meant for immediate use, they can be stored in the same manner as those that under HLD.

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Revised storage requirements

Methods of protecting endoscopes that have undergone HLD and liquid chemical sterilization (LCS) from damage and contamination during storage have been researched and the findings from this research resulted in new recommendations in Section 11.2  Storage of high-level disinfected or liquid chemically sterilized endoscopes. Two types of storage cabinets are being recommended for HLD- and LCS-processed endoscopes: drying cabinets, which are closed cabinets designed for storage of flexible endoscopes that circulate HEPA-filtered or instrument air through the cabinet and each endoscope channel at continuous positive pressure, and conventional cabinets, which are closed cabinets that circulate HEPA-filtered or instrument air through the cabinet at continuous positive pressure, but do not include forced air through endoscope channels. There is no clear consensus at this time among professional organizations regarding which type of cabinet is best; however, drying cabinets have been shown in scientific studies to reduce the risk of retained moisture and microbial contamination.  Endoscopes hung in HEPA-filtered storage cabinets that do not have drying capabilities should be dried prior to storage.

The updated ST91 recommends that cabinets be located in a secure location, such as in the clean workroom and not within the endoscopy procedure room. Storage cabinets should have doors that are kept closed and the cabinets should be located at least three feet from any sink to prevent endoscopes from becoming contaminated by water. It is further recommended that cabinets and endoscopes be visually inspected to ensure cleanliness when the endoscope is placed into the storage cabinet and also when the endoscope is removed for patient use. Storage cabinets should be cleaned following the manufacturer’s IFU, or at least weekly and when visibly soiled.

New annexes for ST91 

  The following annexes have been added to ST91:

  • Annex A Alternatives for keeping cool in the processing environment
  • Annex B Purchase considerations in selecting AERs and LCSPSs
  • Annex C Reference material for repairs
  • Annex D Manufacturer’s written instructions for use (IFU) conflict management
  • Annex E Endoscope visual inspection-Provides information and photos of common scope damage
  • Annex F User verification of cleaning processes
  • Annex G Effects of simethicone on flexible endoscopes
  • Annex H Safety considerations for high-level disinfectants and liquid chemical sterilants
  • Annex I Endoscope microbiocidal methods
  • Annex J Endoscope storage risk assessment
  • Annex K Endoscope drying


Susan Klacik BS, FSC, ACE, CHL, CIS, CRCST, AAMIf, HSPA Clinical Educator, is the HSPA voting representative to AAMI, a role she's held since 1997, and a member of the AORN Guidance Advisory Board. With more than 35 years of healthcare sterile processing experience, Klacik has authored numerous articles and served as a contributing author to the HSPA Central Service Technical Manual, Instrument Resource Manual, Endoscope Reprocessing Manual and Central Service Leadership Manual.  She is the author of the HSPA (formerly IAHCSMM) magazine’s column “Inside Washington” and the OR Manager column “Sterilization and Infection Prevention."