In this In Focus segment we explore the microbiological retention efficacy required for filters in high-risk healthcare environments, the standard and testing process and certification of T-safe as a sterilising grade membrane filter.

• What is a ‘sterilising grade’ membrane filter?
• ASTM F838 standard,
• ASTM F838 testing process,
• T-safe Water Filtration certification.

What is a Sterilising Grade Filter?
A sterilising grade filter is one that can effectively remove and retain all living micro-organisms such as bacteria, fungi and protozoa from a liquid. Such filters use a size determining selective barrier known as a membrane to separate the filtered and unfiltered liquid. The membrane and the subsequent grade of filtration that is provides is rated by its pore size, which is defined as the smallest free passage between both liquids. In the case of filters used for control of waterborne pathogens, a pore size rating of 0.2 µm is required to achieve the FDA definition of a sterilising grade filtration¹ proven through ASTM F838 (Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilised for Liquid Filtration)².

ASTM F838 Standard
ASTM F838, originally developed for liquid filtration in the medicine industry, is the standard commonly applied in the selection of a suitable point of use filter for the prevention of waterborne pathogen exposure from tap and shower outlets in high-risk healthcare environments. To be classified as a sterilising grade filter, a filter must demonstrate retention to 1*10⁷ (10 million) CFU of Brevundimonas diminuta (Br. diminuta) per cm² effective filtration area under specific operating conditions (equivalent to >7 Log or 99.99999%) using ASTM F838.

ASTM Testing Process
The ASTM F838 testing process involves challenging a filter with a liquid suspension of the test organism Br. diminuta(ATCC19146) at a concentration of 10⁷ CFU per cm² effective filtration area, using a purpose-built test rig. Each filter is challenged at least in duplicate and the test is conducted by filtering the required filtration volume through the filter and an attached cellulose nitrate membrane. At the end of the filtration cycle, the cellulose nitrate membrane is transferred onto agar plate for incubation. At the end of the incubation stage, each agar plate is inspected for the identification and enumeration of any residual presence of the test organism.

T-safe Certification
Each individual product model of the T-safe Medical and Pro Water Filtration range has been validated and certified as a sterilizing grade filter for liquid filtration, as it has proven filtration efficacy against Br. diminuta in accordance with ASTM F838-15a. In independent testing conducted by Nelson Laboratories (an ISO 17025 accredited laboratory), T-safe filters exceeded the requirements of a sterilising grade definition proven through ASTM F838 by demonstrating complete retention of Br. diminuta with ‘0’ recovery of the test organism when challenged with up to 4 times of the required concentration of 10⁷ CFU per cm² effective filtration area, equivalent to >10.5 Log reduction with regard to the total challenge concentration^3-5. This means that T-safe provided total retention of the test challenge organism beyond Log 7.

View the T-safe Sterilising Grade Membrane Filter Declaration of Compliance here

References:

1. 1. Guideline on Sterile Drug Products Produced by Aseptic Processing. Technical guideline, FDA, Bethesda, MD, USA, 1987.
2. 2. ASTM F838-15a – Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Water treatment. Test standard, Oct 2015.
3. 3. Stutz B. ASTM F838-15a microbial retention GLP report for T-safe Medical Shower Filter + Tap Filter 92 days. Scientific report, Nelson Labs, Salt Lake City, UT, USA, 2019.
4. 4. Booth H. ASTM F838-15a microbial retention GLP report for T-safe Medical Tap Filter 31 days LF. Scientific report, Nelson Labs, Salt Lake City, UT, USA, 2020.
5. 5. Booth H. ASTM F838-15a microbial retention GLP report for T-safe Medical Tap Filter 31 days SF. Scientific report, Nelson Labs, Salt Lake City, UT, USA, 2020.
6. 6. Stutz B. ASTM F838-15a microbial retention GLP report for T-safe Medical In-Line Filter + Easy-Fix Wall Shower Filter. Scientific report, Nelson Labs, Salt Lake City, UT, USA, 2019.

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