Reprocessing of medical devices: Disinfection validation

Reprocessing of medical devices: Disinfection validation

In the previous article on cleaning validation, it was explained that, in principle, two methods are available for cleaning medical devices: automated cleaning in a washer-disinfector (WD) and manual cleaning. This basic distinction applies in the same way to disinfection, which can be carried out both manually and by machine.
This article outlines the conditions under which the respective disinfection methods are applied and presents the underlying regulatory framework.

Regulatory background

Thorough and effective cleaning is an indispensable prerequisite for successful disinfection and/or sterilisation. Inadequately cleaned medical devices can significantly impair the effectiveness of subsequent disinfection or sterilisation processes and may pose a risk to patient safety. Disinfection can be used either as an intermediate step in the reprocessing cycle or as a final treatment step.

The decision as to which reprocessing methods are suitable and necessary for a specific medical device depends on the respective device category. Responsibility for defining, describing and validating appropriate reprocessing procedures lies with the manufacturer of the medical device (see ISO 17664-1).

Medical devices can be classified using different approaches. In practice, however, the Spaulding classification has become particularly well established. In addition, there are other standards and guidelines that describe criteria and methods for classifying medical devices, including ISO 15883-4, ISO/TS 17665-3, EN 16442 and AAMI/TIR 12.

Semi-critical and critical medical devices are additionally subdivided—based on the required level of reprocessing effort—into devices without special requirements (Group A) and devices with increased requirements (Group B).. Critical medical devices can further be assigned to Group C if particularly high reprocessing requirements apply.

Increased reprocessing requirements may be driven by both design- related and functional factors. For example, lumens, complex geometries, or limited accessibility can make it more difficult to visually verify the effectiveness of cleaning.

The classification of a medical device must therefore always be risk-based and product-specific. This classification is a key factor in determining whether disinfection is considered an intermediate step or the final step in reprocessing. According to KRINKO recommendations, for non-critical and semi-critical medical devices, disinfection may be sufficient as the final procedure.. For critical medical devices, however, sterilisation is generally required as the final step in reprocessing.

*(Hygiene requirements for the reprocessing of medical devices (recommendation of the Commission for Hospital Hygiene and Infection Prevention (KRINKO) at the Robert Koch Institute (RKI) and the Federal Institute for Drugs and Medical Devices (BfArM), 2012)

In the US regulatory environment, medical devices are likewise categorised as non-critical, semi-critical, and critical in accordance with the Spaulding classification.. In addition, AAMI TIR12:2020 subdivides the disinfection process into three levels of effectiveness:

• Low-level disinfection
• Intermediate-level disinfection
• High-level disinfection

For each of these disinfection levels, AAMI TIR12 defines specific acceptance criteria that must be applied – and met – as part of disinfection validation.

When used as an intermediate step , disinfection primarily serves to protect the reprocessing personnel by reducing microbial contamination prior to further handling steps . The required level of disinfection is determined by the respective Spaulding classification. The following table shows which disinfection level is required depending on the criticality of the medical device.

Differences between thermal and chemical disinfection

In principle, disinfection methods can be divided into manual  procedures – predominantly based on chemical disinfection – and automated procedures, in which, thermal, or chemo- thermal processes are used. Thermal disinfection in washer-disinfectors (WDs) is generally preferred over purely chemical disinfection, as it offers higher process reliability, better reproducibility, and more dependable efficacy. This is also consistent with the recommendations of the Commission for Hospital Hygiene and Infection Prevention (KRINKO).

The different disinfection methods – chemical versus thermal – are each based on specific assessment and testing concepts, which must be taken into account when defining the process and during validation.

Automated Process

For thermal disinfection (automated process),the so-called A₀ concept is applied. This concept is based on the assumption that maintaining a defined temperature for a specified period of time results in a predictable lethality for a standardised population of microorganisms. The relationship can be described mathematically as follows:

Here, A is defined as the time equivalent in seconds at a reference temperature of 80 °C required to achieve

a specified disinfection efficacy. The term A₀ is used when a temperature of 10 °C is assumed for the z-value. Δt describes the time interval considered, while T indicates the temperature during  that disinfection interval (ISO 15883-1).

ISO 15883-1 specifies various permissible time-temperature combinations that can be used to calculate and apply the A₀ concept. For example, an A₀ value of 600 can be achieved by disinfection at 80 °C for 10 minutes (600 s), at 90 °C for 1 minute, or at 70 °C for 100 minutes, and so on.

The lower temperature limit for the application of the A₀ concept is 70 °C. Below this threshold, the z-value is assumed to change significantly, and microbial proliferation may potentially occur.

The application of the A₀concept requires compliance with various conditions, such as the load configuration and temperature profile, the type and extent of microbial contamination, and—for example—the presence of process control functions in the washer-disinfector (WD).

The process is considered successful if the recorded A₀ value falls within the minimum and maximum limits defined during validation.

In addition, biological challenge tests may be used when establishing disinfection process in order to experimentally demonstrate the effectiveness of thermal disinfection.

Manual Process

For chemical disinfection (manual process), it must be ensured that each disinfectant used is validated for the intended application and is applied under the manufacturer’s specified conditions of use. Disinfection of the load is considered achieved when the defined parameters for disinfectant concentration, temperature and contact time have been met for all load carriers. These parameters must be defined and adhered to in accordance with the disinfectant manufacturer’s instructions (ISO 15883-1).

The efficacy of chemical disinfectants and antiseptics can be tested in several phases. In phase 1, the basic antimicrobial efficacy against defined microorganisms is assessed without taking practical application conditions of use into account. In phase 2, the intended use is simulated; this phase is subdivided into stage 1 and stage 2.

The disinfectant efficacy on test pieces is assessed based on the level of microbial reduction achieved during Phase 2, Step 2. Therefore, for the validation of medical devices, Phase 2, Step 2 is the relevant stage.

Disinfection validation – efficacy

As part of disinfection validation, the previously cleaned medical device is deliberately contaminated with a defined number of microorganisms. The disinfection procedure to be validated is then performed under specified process conditions in order to demonstrate its efficacy.

Microorganisms differ considerably in their intrinsic – i.e. inherent – , resistance to disinfection procedures. Against this background, the selection of suitable test organisms must be carefully justified and comprehensively documented. Likewise, the selection of the disinfectant depends on the intended clinical use of the medical device and the resulting microbial challenge. Depending on the test organism and the application, a microbial reduction factor of 10⁵ is typically required, corresponding to a five-log reduction (5-log reduction). These requirements are defined, among other sources, in the recommendations of the Commission for Hospital Hygiene and Infection Prevention (KRINKO) at the Robert Koch Institute, in cooperation with the German Federal Institute for Drugs and Medical Devices (BfArM).

In contrast to sterilisation – which aims to complete inactivation of  all viable microorganisms including spores, disinfection is intended to achieve a targeted reduction of defined groups of microorganisms. After completion of the disinfection process, the remaining microbial count on the medical device is determined and the achieved reduction is evaluated. If the predefined microbial reduction is achieved, the disinfection procedure is considered effective and validated.

For terminal disinfection of semi-critical medical devices, the disinfection procedures used in the European Union must be proven to be bactericidal (including mycobacteria), fungicidal and virucidal. In the EU, the selection of test methods for chemical disinfectants is performed in accordance with the applicable standards, taking into account the intended use and mechanism of action. In the United States, by contrast, the relevant test organisms and acceptance criteria are primarily defined by the required disinfection level according to AAMI TIR12.

In summary, regulatory requirements for disinfection validation vary depending on the target market and the product classification. For critical medical devices, both regulatory frameworks require sterilisation as the final reprocessing step.

Disinfection validation – rinsing validation

As part of disinfection validation, it is necessary not only to demonstrate microbiological efficacy but also ensure that no health-relevant residues of cleaning or disinfecting agents, or their reaction products, remain on the medical device. For this reason, the rinsing step following the cleaning and disinfection procedure is of central importance.

Rinsing is required to ensure compliance with defined and acceptable limits for potentially toxic residues. Its effectiveness is significantly influenced by parameters such as rinsing time, temperature, and the volume of water used. In practice, verification that residues can be excluded at concentrations hazardous to health is often performed using a cytotoxicity test ISO 10993-5. This test confirms that the reprocessed medical device does not cause cytotoxic effects during intended use and therefore that biological safety is ensured.

Outlook

Disinfection is a key element of medical device reprocessing and requires a risk-based selection of the method as well as standards-compliant and reproducible validation. The type of medical device, its intended use, and the applicable regulatory framework determine whether disinfection can be applied as an intermediate step or as the final step. In addition to demonstrating efficacy, it is also necessary to exclude health-relevant residues.

At the same time, it is clear that disinfection – even when proven effective against defined groups of microorganisms – does not ensure complete sterility. For critical medical devices in particular, disinfection is therefore insufficient to achieve the level of safety required for patient protection. In such cases, validated sterilisation as a final reprocessing step is mandatory.

The next article will therefore focus on the sterilisation of medical devices. Building on the fundamental presented here, it will examine relevant sterilisation methods, their mechanisms of action, regulatory requirements and the principles of sterilisation validation.

Please note that all details and listings do not claim to be complete, are without guarantee and are for information purposes only.