ST108 Water Testing and Gap Analysis: A Practical Guide for Sterile Processing

Author
Will Fitzgerald, Water Safety Business Manager

ST108 water testing is the routine chemical and microbial monitoring that ANSI/AAMI ST108:2023 calls for to show that the water used to process reusable medical devices stays within specification. For Critical Water, the water used for final rinses and steam generation, that means checking conductivity daily and testing pH, hardness, alkalinity, bacteria and endotoxin at least monthly. An ST108 gap analysis compares your current water system, testing and records against those expectations and turns the differences into a prioritized action plan.

This guide covers both. It lists the limits and minimum testing frequencies from the standard, explains how to read the results, and walks through a gap analysis that hospitals and ambulatory surgery centers (ASCs) can act on. It was written by the water treatment and testing team at Apex Water + Process, which designs, services and tests sterile processing water systems.

What is AAMI ST108?

Like all AAMI documents, ST108 is voluntary unless a regulator adopts it. Accreditation surveyors, however, increasingly use it as the reference point when they review how a sterile processing department (SPD) manages water. It applies wherever reusable devices are processed: hospital SPDs, ASCs, endoscopy and GI suites, and offsite reprocessing centers.

The three ST108 water categories

• Utility Water is water as it comes from the tap, treated further if needed. It is used for flushing, washing and intermediate rinsing.
• Critical Water is extensively treated water. Treatment always includes reverse osmosis (RO) and/or deionization (DI), plus storage, distribution and usually final filtration to control bacteria and endotoxin. It is used for the final rinse of critical devices before sterilization, the final rinse after high-level disinfection (HLD), and steam generator feedwater.
• Steam is vaporized water from a central boiler or a local generator. It is tested as condensate collected at the sterilizer.

Using Critical Water for every stage is not required. ST108 notes it can be unnecessary, costly and harder on equipment. Sterilizer vacuum pump water, for example, should be Utility Water.

ST108 water quality limits for routine monitoring

These are the action levels from ST108 Table 4. Routine results should not exceed them.

* Steam from a dedicated local generator should have condensate pH of 5.0–7.5. Boiler-treated house steam is usually kept at 7.5–9.2 to protect distribution piping.
** When Utility Water is the final rinse after chemical HLD, bacteria should be < 10 CFU/mL and endotoxin < 10 EU/mL. Source: ANSI/AAMI ST108:2023, Tables 2 and 4.

ST108 also recommends trending your results to set facility-specific alert levels below these action levels. A result outside the alert range is an early warning to investigate. A result above the action level means the water is out of specification.

Additional tests during performance qualification

Before a new or substantially modified system goes into routine use, ST108 calls for performance qualification (PQ) against the full Table 2 specification. For Critical Water that adds:

• Total organic carbon (TOC): < 1.0 mg/L
• Metals: aluminum, copper, iron, manganese and zinc, each < 0.1 mg/L
• Anions: chloride, nitrate, phosphate, sulfate and silicate, each < 1 mg/L

PQ typically starts with intensive daily sampling for a defined period. Samples cover the incoming water, the outlet of each treatment step and each point of use. Testing then continues at a reduced frequency for at least one calendar year to capture seasonal changes in the source water. Apex’s practice is to repeat the TOC and ionic panel annually, and again after major repairs, new equipment connections or any loss of control.

How often should SPD water be tested under ST108?

The table below combines ST108 Table 5 (at the water generation system) and Table 6 (at the point of use). These are minimum frequencies.

Source: ANSI/AAMI ST108:2023, Tables 5 and 6.

ST108 calls for more frequent testing while a problem is being resolved. Your risk assessment may also justify it at rarely used points of use or near a dead leg. Re-test after supply interruptions, boil-water notices, construction, repairs and disinfection of the distribution system.

Sample handling matters

Critical Water and steam condensate results are easily skewed by poor collection. Fill bottles to the brim, seal them, and let hot samples cool before testing. Test pH within 15 minutes, because carbon dioxide absorbed from the air lowers it. Collect bacteria and endotoxin samples in sterile, endotoxin-free containers and get them to the laboratory within its holding time. Many apparent failures are actually sampling errors, so confirm the technique before you start tearing into the system.

How to run an ST108 gap analysis in seven steps

1. Form the multidisciplinary team. ST108 expects SPD, Facilities, Infection Prevention, clinical or biomedical engineering, senior leadership and a water treatment specialist to share responsibility. Name an owner for sample collection, results review, maintenance, alarm response and corrective action.
2. Map the water system as it is installed today. Use a current process flow diagram (PFD) that shows incoming water, pretreatment, RO/DI, storage, the distribution loop and every connection. Check that sample ports exist at the loop out after final filtration and just before the return to the tank, since ST108 says they shall. Confirm the loop runs at 3–5 ft/s. Look for dead legs, meaning branches longer than 3–5 times the pipe’s internal diameter.
3. Match every point of use to a water category. List each washer-disinfector, cart washer, ultrasonic cleaner, sink, automated endoscope reprocessor (AER) and sterilizer, and record the water each one receives and the water it should receive. Check the device, equipment and detergent instructions for use (IFUs), and resolve any conflicts with the manufacturers. Manual final rinses after HLD should use Critical Water. AER final rinse water follows the AER manufacturer’s IFU.
4. Compare current testing against Tables 5 and 6. Check every parameter, location and frequency in the tables above against what you actually test today, including steam condensate and point-of-use samples.
5. Review maintenance and disinfection records. ST108 names monthly disinfection of the loop and storage tank as best practice. It also calls for disinfection after a probe, sensor or pump is replaced, plus RO cleaning, filter and membrane changes on the OEM schedule, and a daily system checklist.
6. Test your response and documentation. Confirm that alert and action levels are written down, that out-of-spec results trigger a defined response, and that contingency plans exist for boil-water notices, shutdowns and construction.
7. Prioritize and assign. Rank each gap high, medium or low by patient and operational risk. Give each one an owner, a due date and the evidence needed to close it.

Apex’s free ST108 Compliance Self-Audit Checklist follows this structure and adds a summary report page for leadership.

Gaps we find most often

• No endotoxin testing, or bacteria and endotoxin tested only at the system and not at points of use.
• Daily conductivity treated as proof that the water is Critical Water.
• No steam condensate testing.
• Direct-feed piping with no recirculating loop, dead legs, or no loop-return sample port.
• Action limits copied from the standard, but no trend-based alert levels.
• Disinfection performed only after a failed result.
• Retesting until a sample passes, with no root cause investigation.

What to do when a result is out of specification

ST108 is direct about this. If Critical Water exceeds the bacteria or endotoxin limit, it is no longer qualified as Critical Water and should not be used for the final rinse of critical devices. Use an alternative source that meets specification until the problem is fixed.

1. Notify SPD leadership, Infection Prevention and Facilities right away.
2. Assess impact. Decide which equipment, processes and device loads are affected, and whether processing should pause or switch to an alternate water source.
3. Investigate the root cause. A failure at one point of use usually points to that connection, a dead leg or sampling technique. A failure at the loop out or across the system points to the tank, filters or treatment equipment. Check recent service, disinfection dates and changes in the incoming water.
4. Correct the cause, for example by disinfecting the loop and tank, replacing final filters, repairing or rerouting piping, or servicing the RO.
5. Verify with follow-up samples before returning to normal operation.
6. Document each step, including who decided what and when.

Instrument symptoms that point to a water problem

Spotting, staining and corrosion are often the first signs that water quality has slipped. ST108 Annex I links common symptoms to likely causes. Use the table to decide which tests to run first.

Adapted from ANSI/AAMI ST108:2023, Annex I and Clause 6.3. Detergents, loading and handling can cause the same symptoms, so review those alongside the water results.

Records to keep for surveys

• Multidisciplinary team roster and assigned responsibilities
• Water risk assessment, with review dates
• As-built process flow diagram, posted in the treatment area
• Written sampling plan with locations, frequencies, and alert and action levels
• Laboratory results and trend charts
• Daily conductivity and visual inspection logs
• Maintenance, filter and membrane change, and disinfection records
• Excursion log with investigations, corrective actions and verification results
• Staff training and competency records
• Applicable device, equipment and detergent IFUs
• Contingency plans for boil-water notices, shutdowns and construction

How Apex supports ST108 testing and gap analysis

Apex Water + Process brings together the water treatment, testing and field service experience of Apollo, Aqua Treat, Guardian CSC and Weas Engineering. For sterile processing teams, Apex provides:

• ST108 gap assessments on site, built on the same framework as the Self-Audit Checklist
• Utility Water, Critical Water and steam condensate testing, including bacteria, endotoxin, TOC and ionic panels
• Monthly service programs that include loop and tank disinfection and on-site pH, hardness and alkalinity checks
• Trend reporting to help your team set alert levels and show control to surveyors
• Critical Water system upgrades when the assessment shows equipment or distribution gaps

Start with the ST108 Compliance Self-Audit Checklist , then talk with an Apex water specialist about your results: 1 (844) 603-4077 or info@teamapex.com.