In this insights story, we explore the NETB’s requirements for POU filter validation, the limitations of ASTM F838, and the importance of full life cycle microbial retention efficacy validation.
The NETB 2024/3 is the first definitive guidance for controlling nontuberculous mycobacterium (NTM) in healthcare water systems. It was published on 27 August 2024 as an NHS Estates Technical Bulletin.
The guidance advocates using POU filters as a safeguard against waterborne pathogen infection for specific, high-risk patient groups. It lists a series of factors for Water Safety Groups to consider when undertaking a risk assessment to specify and select a suitable POU filter. It recommends adopting these considerations into the Water Safety Plan to help ensure the ongoing safety of POU as an infection control tool. This includes factors ranging from the robustness of the component manufacture and casing, compatibility with outlets and available activity space, to compatibility with water treatment regimens and operational procedures such as cleaning and sampling protocols.
Most significantly, the guidance requires that filter manufacturers provide verification of microbial retention efficacy (0.2 µm absolute sterilizing-grade filtration) and, critically, that the filter retains NTM and other waterborne pathogens over the claimed installation life of the product under operational conditions commonly found within healthcare water systems.
Microbial retention efficacy
Waterborne infections pose a serious threat to patient safety in healthcare environments, particularly in ICUs, burn units, transplant wards, and other high-risk areas. POU filters serve as a critical last line of defense, preventing harmful pathogens from reaching patients through taps, showers, and water outlets for medical equipment.
However, not all POU filters are equal, and their effectiveness cannot be guaranteed without robust validation. Microbial retention efficacy validation testing can demonstrate that the filter consistently removes harmful bacteria from the water flow. This is typically performed using ASTM F838, a standardized laboratory bacterial challenge test.
ASTM F838 and its limitations
ASTM F838 is the ‘Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration.’ It is the industry-standard test for evaluating the microbial retention efficacy of POU water filters and other membrane-based filtration systems. The test assesses explicitly whether a filter can retain bacteria of a known size under controlled conditions, ensuring it provides a barrier against microbial contamination. The ASTM F838 test uses Brevundimonas diminuta, a small, rod-shaped bacterium (~0.3 µm in size), to challenge the filter. A filter that passes this test (typically rated as 0.2 µm absolute) can theoretically block Pseudomonas aeruginosa (0.5–3 µm), Legionella pneumophila (0.3–0.9 µm), and other waterborne pathogens. This provides some confidence that the POU filter can prevent bacterial contamination at the point of use.
However, the ASTM F838 test only evaluates bacterial retention at a single point in time under controlled conditions. It does not assess how the filter performs after prolonged use, continuous exposure to bacteria and biofilm, or real-world variations in water quality over its entire life cycle. Without a complete life cycle testing under these conditions, a filter that passes ASTM F838 today could fail in a few weeks or months when exposed to challenges commonly presented in hospital water systems such as variations in pressure, temperature, chemical water treatment, water quality and microbiological load. Additionally, ASTM F838 only tests a filter’s ability to retain Brevundimonas diminuta (~0.3 µm), as it is the smallest waterborne bacterium used for membrane filtration validation. Yet, it has no clinical significance.
Full life cycle microbial retention efficacy validation
Whilst ASTM F838 is an essential benchmark for bacterial retention in POU filters, on a single test is insufficient to ensure long-term filter performance and suitability for use in high-risk healthcare environments.
However, full life cycle microbial retention efficacy validation using specific clinical waterborne pathogens of concern provides a more comprehensive assessment, ensuring real-world reliability over the filter’s entire service life under conditions typically found in healthcare water systems. By testing against these clinically relevant bacteria over the entire life cycle, healthcare facilities can ensure that POU filters provide real-world protection against pathogens most likely to cause patient harm.
Full life cycle testing simulates extended exposure to conditions in healthcare water systems, verifying that the filter remains a reliable barrier for its intended lifespan (e.g., 31 or 92 days).
Some bacteria, like Legionella pneumophila, can exist intracellularly within amoebae, making them harder to remove than B. diminuta. Pseudomonas aeruginosa has biofilm-forming capabilities, which may require additional filtration robustness beyond what ASTM F838 measures. Testing against multiple healthcare-relevant organisms ensures the filter can handle different bacterial sizes, structures, and resistance mechanisms.
In hospitals, reduced water flow can lead to user non-compliance (e.g., staff removing and bypassing the filter) or stagnant water, which increases the risk of biofilm formation and bacteria growth. Full life cycle validation ensures filters maintain consistent flow rates without premature clogging.
T-safe microbial retention efficacy: NTM and other waterborne pathogens
The microbial retention efficacy of T-safe medical water filters against NTM and other waterborne pathogens has been proven through independent simulated laboratory life cycle studies, under conditions commonly found in healthcare water systems.
Trial set up
The purpose of the trial was to test the microbial retention efficacy of T-safe medical water filters when exposed to both a constant and spiked challenge of mycobacterium and other waterborne pathogens, in accordance with the principles of ASTM F838, over their prescribed maximum usable life cycle of 31 days. T-safe Medical Tap and Shower Filters were installed on a purpose-built water system and subjected to constant exposure to high concentrations of Mycobacterium spp. through simulated intermittent daily flushing cycles to replicate everyday use.
On days 1, 10, 20, 30, and 40, the filters were challenged with an additional spiked solution of Mycobacterium spp. via an injection point pre-filter. On day 40, the filters were further challenged with additional pathogens via injection point pre-filters, including S. maltophilia, A. baumannii, S. marcesens, K. pneumoniae, B. cepacia, and S. paucimobilis. Water Sampling of the filtrate water was undertaken from each filter after each spiking episode, both pre-flush and post-flush sampling was included.
Results
T-safe Medical Water Filters demonstrated absolute retention of all test organisms in pre- and post-filtrate samples collected on days 1, 10, 20, 30, and 40 of the trial. The absolute retention of the spiked bacteria at day 40 proves the reliability of the 31-day lifespan of the filters against a range of organisms, including:
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- M. abscessus
- M. abscessus bollettii subspecies
- M. chimaera
- S. maltophilia
- A. baumannii
- S. marcescens
- K. pneumoniae
- B. cepacia
- S. paucimobilis
Conclusion
ASTM F838 is a widely recognized test for bacterial retention efficacy, but it has significant limitations when used as the sole validation method for POU filters in high-risk healthcare settings. Without full life cycle testing, facilities relying on ASTM F838 testing alone may have a false sense of security in infection control, as filters may fail or clog prematurely, leading to bacterial breakthroughs, increased infection risks, and additional filter exchanges resulting in increased labor and total costs.
Full life cycle validation testing
- Protects vulnerable patients from waterborne pathogens
- Ensures long-term microbial retention and filter flow rate effectiveness
- Prevents bacterial breakthroughs and loss of infection control
- Minimizes operational costs by preventing premature filter failure
- Supports regulatory compliance and reduces legal risks.
ASTM validation testing only
- Limited scope does not test retention of clinically relevant pathogens
- Provides no assurance of retention efficacy over service life
- Provides no assurance of adequate flow rate over service life in real-world conditions
NETB 2024/3 highlights the increased risks from exposure to waterborne pathogens that specific patient groups face. It recommends adopting a precautionary approach to protect any patient with a long line in situ, and those undergoing cystic fibrosis, hematology, and transplant treatment.
The guidance clearly distinguishes that ASTM F838 testing should be accompanied by full life cycle microbial retention validation. This renders the traditional industry approach of conducting single point-in-time microbial retention efficacy testing insufficient in demonstrating the suitability and reliability of filters for use in high-risk healthcare environments.
