The ISO 17664 series as well as the FDA guidance for reprocessing medical devices in health care settings specify that processing instructions should include the required quality of any water that is used for preparing solutions or for rinsing. A number of water characteristics contribute to water quality such as the amount of microorganisms, bacterial endotoxin levels, water hardness, and ionic contaminants such as chloride. One especially important consideration in water quality is the presence of bacterial endotoxins; given that, unlike microorganisms, endotoxins are not reliably destroyed by disinfection or sterilization processes4. Consequently, if the water used for cleaning and rinsing has high levels of endotoxins, residual endotoxins might remain on reprocessed devices4. This is a concern for medical devices that will contact the patient’s bloodstream, cerebrospinal fluid, or the anterior chamber of the eye as bacterial endotoxins are the most common cause of pyrogenic reactions4.
Different stages of the reprocessing of medical devices may require a different water quality. Guidance on the selection of water quality for the reprocessing of medical devices can be found in AAMI TIR34 which distinguishes two main categories of water quality: utility water and critical water. It also defines accompanying specifications for each water category. Utility water is defined as water as it comes from the tap if certain criteria are met (e.g. water hardness < 150 mg/mL, < 250 mg/L of chlorides present, see Table 1 of AAMI TIR34). Critical water is more extensively treated to ensure that microorganisms as well as inorganic and organic materials are removed from the water. It should contain less than 10 EU/mL of endotoxins4. It is important to note that endotoxin levels in softened or deionized water will not be less than that of tap water because neither process removes endotoxins. That said, deionized water may be used if endotoxins are not a problem; that is, if the endotoxin level is monitored and equivalent to that expected of critical water4.
Selection of water quality as outlined in AAMI TIR34
Manual pre-cleaning and initial rinse
Utility water is suitable for use in pre-cleaning and rinsing of the device immediately after patient use to remove gross debris.
If detergents used for cleaning must be diluted, utility water may be used as well as long as this is compatible with the cleaning agents used. The detergent manufacturer’s IFU should be consulted.
Rinsing after manual cleaning
Utility water is acceptable for the initial post-cleaning rinses to remove detergent residues and loosened soil. However, for semi-critical or critical devices the final post-cleaning rinse is recommended to be performed with critical water to reduce the likelihood of hard-water deposits or endotoxins on the device.
Automated cleaning by medical washers and washer-disinfectors
Utility water is the minimum acceptable quality.
For wash stages the detergent manufacturer’s IFU should be consulted to see if tap water is recommended or if softened, deionized, or critical water is necessary.
For the other stages the washer or washer/disinfector manufacturer’s IFU should be consulted. For example, some manufacturers recommend that tap water be used for the initial rinse, followed by deionized or purified water for the final rinse of all categories of instruments. However, for critical devices critical water should be used during the final rinse and we would recommend the same for semi-critical devices.
Utility water is the minimum quality of water to be used for ultrasonic cleaning.
Dilutions of liquid chemical high-level disinfectants can be performed using utility or critical water, provided that the manufacturer of the liquid chemical has validated its use for this purpose. That said, critical water is preferred over utility water as inorganic or organic compounds present in utility water can affect the efficacy of the disinfectants.
For devices that undergo liquid chemical sterilization or high-level disinfection it is important that rinsing following disinfection or liquid chemical sterilization not only removes inorganic residuals but also avoids the recontamination of the device with microorganisms or organic residues. Therefore, for these devices sterile or bacteria-free rather than tap water is required. Critical or bacteria-free water should also be used for the post-disinfection rinses of automated endoscope reprocessors.
For devices that are terminally disinfected by thermal disinfection in a washer/disinfector, utility water is acceptable if it meets the washer/disinfector manufacturer’s IFU and if the water has a low hardness level (< 30 ppm). Otherwise, critical water should be used. For thermal disinfection by pasteurization the manufacturer’s written IFU should be followed. Critical devices can be thermally disinfected as an intermediate processing step between cleaning and sterilization. In these cases, critical water should be used. The same is true for semi-critical devices, for which we would recommend the use of critical water as well.
Generally, softened or deionized water is used for steam sterilization, but critical water could also be used.
Given that noncritical devices do not usually pose a risk of pyrogenic reactions or infections in patients, the use of utility water tends to be acceptable for all reprocessing stages.
It is important to consider water quality during all stages of processing. Whenever water is used for diluting detergents or disinfectants, the technical data sheet of these chemicals should be consulted to make sure that the adequate water quality is used. When water is used for thermal disinfection the manufacturer’s IFU should be consulted and the criticality of the devices should be considered. Generally, critical devices require critical water for any reprocessing step that comes after cleaning and we recommend the same for semi-critical devices.
 ISO17664: 2017 Processing of health care products – Information to be provided by the medical device manufacturer for the processing of medical devices.
 ISO 17664-2: 2021 Processing of health care products – Information to be provided by the medical device manufacturer for the processing of medical devices – Part 2: non-critical medical devices.
 FDA Guidance for Industry and Food and Drug Administration Staff: 2015 Reprocessing Medical Devices in Health Care Settings: Validation methods and Labeling
 AAMI TIR34: 2014/(R)2017 Water for the reprocessing of medical devices.