Why Disinfection Remains the Standard for Reprocessing Semi-Critical Flexible Endoscopes

The transmission of pathogenic microorganisms through flexible endoscopes has gained attention in recent years, with concern that improperly reprocessed endoscopes may harbor transmissible microorganisms. To promote patient safety, semi-critical medical devices such as most thermolabile flexible endoscopes need to be properly cleaned and disinfected. Due to the efficacy of modern disinfection processes, additional sterilization is usually not required.

So why is there still so much confusion surrounding sterilization in endoscope washer-disinfectors (EWDs)? And can sterilization in EWDs really be considered effective? Finally, what role does ISO 14937 play in this and how does it differ from ISO 15883-4?

Continue reading to learn more about the differences between disinfection and sterilization, and why liquid chemical sterilization does not sterilize your flexible endoscopes and offers no significant additional safety compared to disinfection.

Due to the complex nature of their design, with numerous lumens and narrow channels, and their thermolabile nature, flexible endoscopes require a reliable and specialized reprocessing protocol to prevent nosocomial infection and support endoscope longevity. There are several important steps when reprocessing flexible endoscopes, including precleaning, manual cleaning, disinfection, drying, storage, documentation, and sometimes, if applicable, sterilization. Each step plays a key role in the reduction of organic load and microbial contamination. In the following article, we will focus on disinfection.

Disinfection: Meeting international reprocessing standards

According to the Spaulding classification, the majority of flexible endoscopes are semi-critical devices because they are in close contact with intact mucous membranes, and they generally do not penetrate any sterile tissue inside the human body. These endoscopes must be cleaned and disinfected so that bacteria, fungi, mycobacteria, and viruses are eliminated/inactivated to a defined level [1]. Therefore, disinfection refers in general to “a process to inactivate viable microorganisms to a level previously specified as being appropriate for a defined purpose” [2]. The above-mentioned spectrum of efficacy of disinfectants needs to be demonstrated with standardized test methods and depends on a sufficient concentration, contact time and temperature. For an automated process, the ISO 15883-4 provides instructions for manufacturers to consider during the development and type testing of endoscope washer-disinfectors (EWDs) [2].

Enhancing disinfection with sporicidal activity

For some endoscopes, the elimination of bacterial spores is important. Bacterial spores are produced by bacteria for survival purposes under certain environmental conditions and are much more resistant to harsh conditions, such as those conferred by some chemicals. However, with a disinfectant that includes sporicidal activity, spore count can be reduced significantly [2][3][8][9][10].

Globally, various regulations exist that govern sporicidal activity of disinfecting chemicals (see table 1). EN 17126 [3], a European standard for manufacturers of disinfectants, describes a quantitative suspension test method to determine the sporicidal activity of disinfectants. Based on this standard, which uses a hypervirulent strain of Clostridioides difficile (formerly Clostridium difficile) termed PCR ribotype 027 as one of the potential test microorganisms, the sporicidal activity of a disinfectant can be demonstrated by a reduction in spore count with a factor of 4 log10. This sporicidal efficacy testing of disinfectants is typically performed under clean and dirty conditions. Testing under dirty conditions using artificial test soil, provides a higher safety margin as it simulates potential residuals from earlier cleaning stages. In addition, the international standard ISO 15883-4 clearly states that a 4 log10 reduction in bacterial endospores must be achieved after a complete standard cycle in an EWD [2].

Table 1: Testing requirements on sporicidal efficacy






(sporicidal activity for sterilant claim)

AOAC-966.04 (sporicidal activity of disinfectant)

EN 17126
(sporicidal activity)

Test species

Bacillus subtilis, Clostridium sporogenes

Bacillus subtilis
Clostrdiium sporogenes

Bacillus subtilis/Bacillus. cereus
Clostridioides difficile

Minimum spore reduction

Not specified

Not specified

4 log10

EWD process



ISO 15883-4

Test species

Reference to regional standards for disinfectants’ activity testing

Minimum spore reduction

4 log10

General sterilization process



ISO 14937

Test species

Potential test organisms:
Bacillus atrophaeus
Clostridium sporogenes
Geobacillus stearothermophilus

Minimum spore reduction

Not specified

The buzz around liquid chemical sterilization and ISO 14937

In contrast to disinfection, sterilization is defined as a “validated process used to render product free from viable microorganisms” [4]. This means that the process of sterilization eliminates microorganisms to near zero, which consequently means that process efficacy is measured as a probability termed SAL (Sterility Assurance Level). Typically, an SAL of 10-6 should be reached [5]. In general, the preferred sterilization method for medical devices is steam sterilization and the effectiveness of this method is well researched. However, it cannot be used for critical reusable flexible endoscopes used inside sterile body cavities due to heat- sensitive components that are easily damaged by the high temperatures [1].

Why liquid chemical sterilization does NOT mean your endoscope is sterilized

In other regions of the world such as the U.S., sporicidal disinfectants are also called sterilizing agents/sterilants, and these are commonly associated with high-level disinfection (HLD). HLD is defined as a “complete elimination of all microorganisms in or on an instrument, except for small numbers of bacterial spores” [6][7]. However, within the context of disinfection, the term sterilant is misleading as disinfection and sterilization are two distinct methods. This becomes clearer when evaluating liquid chemical sterilization (LCS).

LCS was introduced for heat-sensitive devices and uses HLD disinfectants that are termed sterilants based on their sporicidal efficacy, e.g., when tested according to AOAC-966.04 [8] (US) or TGA-104 [9] (Australia) (see table 1). When a device is subject to LCS, it is completely immersed in an active sterilant solution for a prescribed period of time (brand-dependent, up to 12 hours), at a controlled temperature and concentration [6]. However, when used in conjunction with an EWD, there is no specific sterilization standard to comply to. The EWD manufacturers may use the ISO 14937 standard [10]. This international standard specifies general requirements for a sterilization process by physical and/or chemical means of packaged medical devices (in case no other standard for a particular sterilization process exists).

In contrast to ISO 15883-4, ISO 14937 does not specify a number of key variables, including the test method, test microorganisms, and minimum spore reduction (see table 1). In addition, it is important to note that the testing conditions for sterilization efficacy studies differ significantly from disinfection. Sterilization efficacy tests are done under clean conditions only. When it comes to disinfection, compliance testing is simulating clean and dirty conditions.

In terms of defining the term “sterilant”, in Europe, chemicals that are used for automated disinfection are called disinfectants with sporicidal efficacy. In fact, reprocessing of endoscopes within an EWD cannot meet the conditions for sterilization. For example, even if a sterilant is used in an EWD, the water used for final rinsing and the air inside and outside of the machine are not sterile. Furthermore, sterile medical devices require specific preservation processes including appropriate sterile packaging and storage.

Disinfection: An unrivalled reprocessing process

Several attempts to improve disinfection have been made, including methods such as double HLD (DHLD), HLD plus an additional sterilization using ethylene oxide (ETO) gas, and LCS – which is common in the U.S. Despite disinfection remaining the standard for endoscope reprocessing, in 2015, the U.S. Food and Drug Administration (FDA) suggested that implementation of supplemental measurements, such as LCS or DHLD, may increase the safety of reprocessed duodenoscopes [11].

But evidence from recent studies demonstrates that the aforementioned methods do not provide greater hygiene safety levels, when compared with disinfection processes. For example, a study published by Gromski et al [12] compared DHLD with LCS. The results of a hygiene test from almost 900 duodenoscopes resulted in low rates of positive cultures demonstrating that the use of LCS for the reprocessing of duodenoscopes does not improve hygiene or impart any additional safety.

An additional study evidenced that DHLD processes offer no added benefits when compared with standard HLD processes [13]. Moreover, a comparison of duodenoscopes reprocessed by HLD, DHLD, or HLD plus ETO gas sterilization found no significant differences between these groups in terms of bacterial contamination [14].

Taken together, these studies indicate that a routine single disinfection process - the current standard for reprocessing - is effective.


Effective reprocessing is mandatory to ensure medical instruments remain safe to use and to prevent the transmission of pathogenic microorganisms. While attempts are being made to advocate for LCS and DHLD in the U.S., numerous studies have argued that such methods have no significant impact on microbial growth and transmission potential. In fact, there is no published data to date that demonstrates that LCS of endoscopes offers a better outcome over routine disinfection [12].

Indeed, it is important that each step of the reprocessing process, including precleaning and manual cleaning, is carried out properly to ensure a successful disinfection of semi-critical flexible endoscopes – for unrivalled patient safety.

Sources and further readings

  1. Reprocessing of flexible endoscopes and endoscopic accessories used in gastrointestinal endoscopy: Position Statement of the European Society of Gastrointestinal Endoscopy (ESGE) and European Society of Gastroenterology Nurses and Associates (ESGENA) – Update 2018. Beilenhoff, U, Biering, H, Blum, R, et al. European Society of Gastrointestinal Endoscopy (2018) https://www.esge.com/assets/downloads/pdfs/guidelines/2018_a_0759_1629.pdf

  2. ISO 15883-4:2018 - Washer-disinfectors — Part 4: Requirements and tests for washer-disinfectors employing chemical disinfection for thermolabile endoscopes.

  3. EN 17126 - European Standards. https://www.en-standard.eu/csn-en-17126-chemical-disinfectants-and-antiseptics-quantitative-suspension-test-for-the-evaluation-of-sporicidal-activity-of-chemical-disinfectants-in-the-medical-area-test-method-and-requirements-phase-2-step-1/

  4. ISO 11139:2018 - Sterilization of health care products — Vocabulary of terms used in sterilization and related equipment and process standards. https://www.iso.org/standard/66262.html

  5. ISO 17665–1:2006 Sterilization of health care products – Moist heat – Part 1: Requirements for the development, validation and routine control of a sterilization process for medical devices Ref.: Definitions 3.50

  6. A Rational Approach to Disinfection and Sterilization. Guideline for Disinfection and Sterilization in Healthcare Facilities (2008). Centers for Disease Control and Prevention. https://www.cdc.gov/infectioncontrol/guidelines/disinfection/rational-approach.html

  7. Other Sterilization Methods. Guideline for Disinfection and Sterilization in Healthcare Facilities (2008). Centers for Disease Control and Prevention. https://www.cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/other-methods.html

  8. AOAC 966.04-2002, Sporicidal Activity of Disinfectants. http://www.aoacofficialmethod.org/

  9. Therapeutic Goods (Standard for Disinfectants and Sanitary Products) (TGO 104) Order 2019. https://www.legislation.gov.au/Details/F2019L00482

  10. 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. https://www.iso.org/standard/44954.html

  11. Supplemental Measures to Enhance Duodenoscope Reprocessing: FDA Safety Communication. https://www.fdanews.com/ext/resources/files/08-15/081015-duodenoscopes-fda.pdf?1520541508%20.%20Aug%202015

  12. Double high-level disinfection versus liquid chemical sterilization for reprocessing of duodenoscopes used for ERCP: a prospective randomized study. Gromski, M, A, Sieber, M, S, Sherman, S, Rex, D, K. Gastrointestinal endoscopy (2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8101057/

  13. A randomized trial of single versus double high-level disinfection of duodenoscopes and linear echoendoscopes using standard automated reprocessing. Bartles, R, L, Leggett, J, E, Hove, S, et al. Gastrointestinal Endoscopy (2018) https://www.giejournal.org/article/S0016-5107(18)30130-5/fulltext

  14. Randomized Comparison of 3 High-Level Disinfection and Sterilization Procedures for Duodenoscopes. Snyder, G, M, Wright, S, B, Smithey, A, et al. Gastroenterology (2017) https://pubmed.ncbi.nlm.nih.gov/28711629/.