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ST108: The Future of Water for the Processing of Medical Devices Is Coming

One of the things that I enjoy doing (although I don’t get paid for it, so what exactly is wrong with me?) is participate in the AAMI Standards Making Process. I have been, and continue to be, very involved in the development of the replacement for AAMI TIR34:2014 “Water for the reprocessing of medical devices,” and the upcoming standard ST108 “Water for the processing of medical devices.” This article is a briefing on how things will change in this area once ST108 is published (hopefully in 2023).

What’s different between TIR34 and ST108?
For one, ST108 will be an American National Standard. This means that its requirements will be enforceable. This is a good thing if you have had pushback when requesting upgrades to your water supply system. It is a bad thing if you have been accepting less-than-adequate water quality because “it’s always been like that.”

Specific differences are that we, the AAMI ST WG95 working group, have gone far beyond what is in TIR34 to not only tell you what should and should not be in water for processing medical devices, but have provided in-depth information on how to create a good water supply system that can be, and is, maintained to ensure good water quality, and therefore, better patient outcomes. This includes:

  • Who should be responsible for establishment and maintenance of water quality
  • How to apply a risk analysis framework to evaluating your water supply system, including the potential downsides of not providing good-quality water
  • Expansion and clarification of water quality categories for processing of medical devices and where and when each should be used
  • Interaction of water quality with disinfection technologies
  • Effects of water quality on steam and liquid-chemical sterilization
  • Water system design and qualification
  • Routine monitoring activities
  • Continuous quality improvement
  • Maintenance
  • What to do in cases of shutdown, boil water alerts, other interruptions to supply, and after repairs or modification

Many of my clients have had difficulties with water because there was no separation of the municipal supply from what is used in the healthcare institution. Particulates, low or high pH, and contaminants are some of the more prevalent issues. The municipal supply is required to meet the EPA safe drinking water standards. Medical devices are frequently more sensitive to contaminants than people, although if you were to mix up a water sample with the maximum levels of contaminants allowed by the EPA safe drinking water standards, I don’t think you would drink more than a small sip before spitting it out. ST108’s requirements limit the contaminants in safe drinking water most likely to cause problems in medical device processing. TIR34 did this as well, but ST108 presents a clearer picture of what needs to be done and how to do it. Here is Table 2 of ST108, which lists allowable contaminant levels from the draft standard. Note that this is still a draft and some of the numbers may change (and chloride will go to a lower acceptance level if I get my way).

*Values for steam condensate
**For plant/house steam, most boilers should be treated to maintain a condensate pH of 7.5 to 9.2
***If hardness is greater than 150 mg/L, a water softener is recommended unless used for washing, where the cleaning chemistry is capable of handling higher levels of hardness
****When Utility Water is used after chemical high-level disinfection as a final rinse, then bacteria should be <10 CFU/mL and endotoxin should be <10 EU/mL

TIR34’s requirements table is far less comprehensive and lumps steam condensate and critical water together, which does not reflect the reality of house steam vs. “clean” steam (actually “process” steam which is made from critical water). TIR34’s requirement are shown in columns to the right of the ST108 columns in the above table. One thing to note is that there are a lot more contaminants to check in ST108. These things do not belong on instruments and they certainly do not belong in the patient. A specific, important item is that the acceptance level for iron ions in utility water has been cut in half. Iron will initiate corrosion on stainless steel instruments, so it was determined that its presence should be limited to a greater degree than it was in TIR34.

As for steam, house steam is generated from water that is treated with pH adjusters, sulfites, polymers, and amine chemicals to create a steam that will not corrode the black iron plumbing that is typically used in steam distribution systems. In ST108, the acceptance range for pH for steam condensate from a house steam system has been changed to 7–9.2, because that is what a good house steam system will provide. It is not possible to meet the TIR34 acceptance range with house steam. For a “clean” (not really) steam system, you are using critical water to generate the steam in a stainless steel boiler and using stainless steel plumbing to take the steam to your sterilizer that has stainless steel plumbing, chamber, jacket, and valves. There should be little to no change in the steam chemistry from the original critical water used to make the steam to the condensate collected for analysis after the system has cleaned itself out during or after qualification. If there is, you have a problem.

Why do I say “clean” (not really) steam? This is very good quality steam with far fewer contaminants than house steam. But clean steam has a specific definition from the pharmaceutical world. This is closer to that definition, but it is not pharma-grade clean steam.

Now that the line in the sand has been drawn, how do you get there and how do you stay there? ST108 has extensive discussion of how to treat water to achieve and maintain these levels of contaminants. It also creates the structure of a group within the healthcare institution whose responsibility is to ensure that the water quality starts good and stays good. There is extensive information on commissioning water systems and their monitoring and maintenance, information you can use to be an informed consumer of services that may have seemed like black magic in the past.

Knowledge is power. We, the WG95 team, spent a lot of time and energy to help you, the SPD end user, gain that power for the benefit of your patients and your repair budget. Good quality water helps in correct processing and can eliminate a lot of repair bills. While there may be capital involved in getting your water system to a good state, once that is done, the old proverb “an ounce of prevention (water system maintenance) is worth a pound of cure” rings true.


Dr. Jonathan Wilder has worked with all thermal and chemical sterilization methods, as well as cleaning and disinfection methodologies, bringing his background in physical chemistry and surface physics to bear upon difficult problems in the field. He has been an active participant in U.S. and international standards development since 1998 through AAMI. As of January 2018, he is the cochair of the U.S. standards-making committee for hospital steam sterilizers.

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