AAMI ST108 is a standard published by the Association for the Advancement of Medical Instrumentation that defines water quality requirements for the processing of reusable medical devices. It establishes limits for endotoxin, microbial contamination, total organic carbon (TOC), conductivity, and chemical purity to ensure patient safety during instrument reprocessing.

Critical Water Testing | AAMI ST108 Water Standards

ANSI/AAMI ST108 Water Testing for Sterile Processing

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Are you in compliance?

In 2023, AAMI published ANSI/AAMI ST108:2023, Water for the Processing of Medical Devices. It replaces AAMI TIR34:2014 and sets chemical and microbiological requirements for the water used to clean, rinse and steam-sterilize reusable medical devices. Sterile processing departments test three types of water against it: utility water, critical water and steam.

Why water quality matters

Water that does not meet the standard affects medical device processing in three ways:

The device: corrosion, pitting and scaling; biomass build-up; increased microbial load or endotoxin.
The process: detergents work less well, and the water system degrades through biofouling or scaling.
The patient: infection and toxicity.

Even a well-validated cleaning procedure falls short if the water is wrong. Sterile processing staff should watch for the visible signs, such as residue on instruments or poor rinsing, and back them up with routine testing.

ST108 water quality limits

Water quality limits by water type (summary of ANSI/AAMI ST108:2023)
Measurement	Units	Utility water	Critical water	Steam (as condensate)
pH at 25 °C	pH	6.5–9.5	5.0–7.5	5.0–9.2*
Conductivity	µS/cm	<500	<10	<10
Total alkalinity	mg CaCO₃/L	<400	<8	<8
Total hardness	mg CaCO₃/L	<150	<1	<1
Bacteria (HPC)	CFU/mL	<500	<10	N/A
Endotoxin	EU/mL	N/A	<10	N/A
Color and turbidity	Visual	Colorless, clear, no sediment	Colorless, clear, no sediment	Colorless, clear, no sediment

* The steam pH range is wider because steam is often generated centrally and travels through black iron piping, and boiler treatment chemicals can raise the pH. A steam pH below 7.5 should be avoided in these systems.

Ionic contaminants, tested annually

In addition to the routine monthly or quarterly checks, ST108 calls for an annual test of ionic contaminants:

Corrosive: chloride, nitrate, iron, copper, manganese, zinc.
Scale-forming: phosphate, sulfate, silicate, hardness.

Testing schedule

We recommend testing each water system for the parameters that apply to its water type and use. American Analytical provides laboratory analysis for all the analytes ST108:2023 requires.

What we test, by water type and frequency
Water type and frequency	pH, conductivity, alkalinity, hardness	Bacteria (HPC)	Endotoxin	TOC	Ionic contaminants
Critical water, monthly	✓	✓	✓
Critical water, annual	✓	✓	✓	✓	✓
Utility water, quarterly	✓	✓
Utility water, annual	✓	✓			✓
Steam, quarterly	✓
Steam, annual	✓				✓

Endotoxin testing

Endotoxin is a required test for critical water under ST108 (limit <10 EU/mL). It is also run during system validation. After any corrective action, collect a follow-up sample, typically within 24 hours. A single result is a point in time, so routine testing is what shows a trend before it becomes a failure.

Learn more about endotoxin testing

AAMI ST108

Compliant Testing

Healthcare

Facility Ready

SDVOSB

SBA-certified

ISO 17025

ANAB AT-3321

Testing Methodology & Details

Methods Used

Bacterial endotoxin — Limulus Amebocyte Lysate (LAL)
Total Organic Carbon (TOC) — UV/persulfate oxidation
Conductivity — calibrated cell measurement
Microbial enumeration — heterotrophic plate count (HPC)
pH, alkalinity and hardness
Ionic contaminants, including metals by EPA 200.8

Turnaround Time

Standard turnaround is 7–14 business days from sample acceptance, with every parameter in one ST108 report. Rush turnaround is available when agreed before the samples ship.

Compliance

Bacteria (HPC) and metals by EPA 200.8 are within our ISO/IEC 17025 accreditation (ANAB AT-3321). pH, conductivity, alkalinity, hardness, endotoxin, TOC and anions are performed under our quality system and marked on the report as outside the accreditation scope. Reports show each result against its ST108 limit, ready for surveyor review.

Frequently Asked Questions

Q: What is AAMI ST108?

AAMI ST108 is a standard published by the Association for the Advancement of Medical Instrumentation that defines water quality requirements for the processing of reusable medical devices. It sets limits for utility water, critical water and steam, including pH, conductivity, alkalinity, hardness, bacteria (HPC) and endotoxin, plus annual checks for TOC and ionic contaminants.

Q: Who needs critical water testing?

Hospitals, ambulatory surgery centers, dental clinics, and any healthcare facility that reprocesses medical devices needs critical water testing. Surveyors look for documented water testing against AAMI ST108. Contact us at (928) 985-9399 to set up a monitoring program.

Q: What parameters are tested?

pH, conductivity, alkalinity, hardness and bacteria (HPC) on every routine sample; endotoxin for critical water; and TOC and ionic contaminants once a year. Each report shows pass or fail against the ST108 limit for that water type.

Q: What is the turnaround time?

Standard turnaround is 7–14 business days from sample acceptance, with every parameter in one ST108 report. Rush turnaround is available when agreed before the samples ship.

Q: Do you provide compliance documentation?

Yes. Every critical water report includes a formal compliance summary referencing the specific AAMI ST108 limits, pass/fail determinations, and trend data when part of a monitoring program. It is formatted for Joint Commission, CMS and state health department surveyor review. Call (928) 985-9399 for more information.

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AAMI ST108 — 2026 Compliance Update

Reviewed for accuracy: November 2026. ANSI/AAMI ST108:2023 remains the current standard for water used in medical device processing. Joint Commission and CMS surveyors continue to expand documentation expectations in 2026, including monitoring frequency, trend analysis, and demonstrated corrective-action workflows when limits are exceeded.

Recent guidance from AAMI clarifies expectations for ambulatory surgery centers and dental settings — facilities that historically relied on tap water for instrument processing now need documented critical water programs. AATLS provides ISO/IEC 17025 accredited endotoxin (LAL), TOC, conductivity, HPC, and chemical purity testing aligned to the ST108 specifications, with reports formatted for surveyor review.

Considering a monitoring program for 2026? Our team can advise on sampling frequency, point-of-use vs storage-tank testing strategies, and the documentation package needed for survey readiness. Call (928) 985-9399 or use the form below.