Before looking into the validation requirements of thermal disinfection, let’s first go a bit more into what thermal disinfection is and why you should consider having a thermal disinfection procedure in place.
What is thermal disinfection?
Thermal disinfection is defined in the ISO15883-1:2006 standard as, ”disinfection achieved by the action of moist heat.” This can be performed by exposing devices to hot water, steam, or a combination of the two. Thermal disinfection can be a part of the automated cleaning program set in washer disinfectors. In this case, temperatures ranging from 82°C to 95°C will typically be used with the containment time depending on the model of the washer disinfector2. When it is not part of an automated cleaning program, thermal disinfection can occur through boiling test items or through pasteurization. The latter typically uses water at temperatures of 65°C to 77°C for a contact time of at least 30 minutes2.
Should I include a thermal disinfection process in my IFU?
Both ISO15883-1:20061 and AAMI TIR12:2020 offer guidance on this issue, ‘Thermal disinfection of devices, as a final step in processing or to prepare a device for safe handling, is recommended whenever possible.” Hence, if your materials are resistant to heat, distortion, warpage, and corrosion a thermal disinfection process is recommended. In our experience, European regulatory agencies demand the inclusion of thermal disinfection in the automated cleaning program if automated cleaning is part of the IFU. The advantage of automated thermal disinfection is that the process is more easily controlled and reduces hazards to the staff, patients, and the environment as compared to using chemical disinfectants1. Consequently, even when your product is on the market outside of Europe and another terminal processing step is in place such as steam sterilization, including a thermal disinfection process could be considered to warrant healthcare personnel safety and an added safety factor to the processing steps.
Whereas thermal disinfection will often be an intermediate processing step, it can be the terminal processing step as well. An example of devices that will often be terminally processed using thermal disinfection are respiratory devices. Additionally, home-care devices such as breast pumps are also thermally disinfected.
Should I validate the thermal disinfection process?
AAMI TIR12:20202 and ISO17664:2017 state that if the thermal disinfection step is part of the IFU, a validation must be performed. This validation can be performed in different ways.
1) The A0 method
The A0 method can be used to validate automated thermal disinfection processes. The principle is that for a moist heat disinfection process, a particular time at a particular temperature is expected to have a predictable lethal effect against a standardized population of organisms1. Therefore, the measured temperature in or on the device can be used to determine if the desired accumulated lethality value, or A0 value, is reached. Consequently, A0 is a physical parameter denoting the inactivation of microorganisms.
The mathematic formula for calculation of A0 is: A0 = Σ10(T-80)/z x ∆t, with ∆t the selected time period in seconds, T the temperature of the load in °C and z = 10°C . ISO15883-1:20061 specifies that an A0 value of 60 is the minimum acceptable value for products which contact intact skin, whereas A0 values of 600 should be achieved for critical or semi-critical devices. Thermal disinfection parameters that meet this A0 = 600 requirement are for example exposure at 80°C for 10 minutes, 90°C for 1 minute or 93°C for 30 seconds. However, specific countries may have additional requirements, such as Germany, where A0 = 3000 is required for critical and semi-critical devices. This corresponds to, for example, 80°C for 50 minutes, 90°C for 5 minutes, or 93°C for 2 minutes and 30 seconds.
When validating according to the A0 method, it is important to keep in mind that the temperature sensor positioning should be based on relevant, worst-case parameters such as materials, design characteristics, geometry, etc.
2) Microbiological challenge studies
Microbiological challenge studies can be used to validate both automated and non-automated thermal disinfection processes. When evaluating automated thermal disinfection processes, glass ampoules carrying microorganisms are used to demonstrate the process lethality provided by the disinfection cycle. Depending on the Spaulding classification of the device low-level, intermediate-level, or high-level disinfection may be appropriate, which will result in the use of different test organisms and method of evaluation used. For non-automated thermal disinfection, direct inoculation will be used.
Which validation method should I choose?
For automated thermal disinfection processes European regulatory agencies expect A0 data as indicated in ISO15883-1:20061. US FDA on the other hand does not accept A0 data and will instead recommend microbiological challenge studies. However, in our experience US FDA will only request these data when thermal disinfection is the final reprocessing step. When thermal disinfection is not the final reprocessing step, performing a thermal disinfection validation is not required. This contrasts with Europe, where regulatory agencies expect A0 data even when automated thermal disinfection is not the terminal reprocessing step—to accommodate user safety. Non-automated thermal disinfection processes are validated using microbiological challenge studies.
Requirements for thermal disinfection instructions and validations will vary on whether your product is marketed in the EU, US, or in both markets. If your product can withstand thermal disinfection, it is either required to be part of the automated cleaning process (EU) or strongly recommended (US) and will have to be validated using the A0 method (EU) or can be validated with microbiological challenge studies depending on whether or not it’s the terminal processing step (US).
 ISO15883-1:2006: Washer-Disinfectors Part 1: General requirements, terms and definitions and tests
 AAMI TIR12:2020: Designing, testing, and labeling medical devices intended for processing by health care facilities: A guide for device manufacturers
 ISO17664:2017: Processing of health care products — Information to be provided by the medical device manufacturer for the processing of medical devices
 The z value is a measurement in °C of the temperature relationship to the killing process. Bacterial spores, which are the most resistant of all microorganisms, have an average value of z = 10°C. More information on the A0 concept can be found in ISO15883-1:2006: Washer-Disinfectors Part 1: General requirements, terms and definitions and tests
 Rosenberg, Urs 2003 Discussion: Thermal Disinfection – The A0 concept and the Biological Background, Zentralsterilisation – Central Service Volume 11.