If you work in pharmaceutical or biotech manufacturing, you have likely encountered both 0.2 micron and 0.22 micron pore size ratings on sterilizing grade filters. The difference between these two numbers causes real confusion during filter selection and validation planning.
The short answer is that both ratings describe filters designed to achieve the same bacterial retention performance. The longer answer involves understanding how pore size ratings are determined, why manufacturers adopted different labeling conventions, and what actually matters when you validate a sterilizing filter for your process.
This article walks through each of those questions so you can make more informed decisions about filter selection and validation in your facility.
The 0.2- and 0.22-micron ratings became established sterilizing-grade filtration standards through scientific testing and industry validation. Historically, larger pore sizes, such as 0.45 microns, were found to allow certain smaller bacteria, including Brevundimonas diminuta, to pass through. Research demonstrated that a tighter 0.22-micron membrane could consistently retain this challenge organism, leading to its widespread adoption as the standard for sterilizing-grade filtration. The 0.2-micron designation is commonly used interchangeably with 0.22 microns, with both representing essentially the same filtration performance and accepted sterilizing-grade standard.
A pore size rating on a membrane filter is not a direct physical measurement of individual pore openings. Membrane pores are irregular, interconnected pathways through the filter media. The stated rating represents the filter's ability to retain particles and microorganisms of a defined size under standardized test conditions.
The bubble point test is the primary method used to characterize membrane pore size. This test pushes pressurized air through a wetted membrane and measures the pressure at which air first passes through the largest pores. Sterilizing-grade ratings are determined by the bacterial challenge, and the bubble point specification is then linked to that result through testing, serving as a non-destructive integrity check.
Because different filter media manufacturers developed their testing methods independently, some adopted 0.2 micron and others adopted 0.22 micron as the designation for what is functionally the same retention capability.
The 0.22-micron rating is a more precise numerical measurement than 0.2 microns because it includes an additional decimal place. When rounded to one decimal place, 0.22 becomes 0.2. Therefore, the two values are often used interchangeably in the filtration industry, but 0.22 microns provides a more specific expression of the nominal pore-size rating rather than indicating a tighter filtration standard.
The existence of two ratings traces back to how different companies defined their sterilizing grade membranes. When Brevundimonas diminuta was identified as the standard challenge organism, filter manufacturers independently designed membranes capable of retaining this organism.
The minimum challenge level is 107 colony forming units per cm2 of effective filtration area. Manufacturers that met this threshold assigned their own pore size designation to the qualifying membrane.
Some manufacturers labeled their qualifying membranes as 0.2 micron rated. Others labeled theirs as 0.22 micron rated. The difference in the number reflects the rating convention each manufacturer selected, not a difference in the membrane's ability to retain bacteria. Both designations describe filters that meet the same sterilizing filtration performance threshold.
The true measure of a sterilizing grade filter is its ability to pass the bacterial challenge test described in ASTM F838. This test evaluates whether a membrane can retain at least 107 CFU/cm2 of B. diminuta with zero passage of organisms to the filtrate.
A filter that passes this challenge qualifies as sterilizing grade, regardless of whether it carries a 0.2 or 0.22 micron label.
The PDA Technical Report No. 26 (Revised 2025) provides a systematic approach to selecting and validating sterilizing filters. This guidance emphasizes that filter qualification by the manufacturer is only the starting point.
Process-specific validation with your actual drug product, under your operating conditions, is what ultimately demonstrates that a filter will perform as required in your facility.
The membrane polymer affects chemical compatibility, protein binding, flow characteristics, and wettability. Common membrane materials for pharmaceutical sterilizing filtration include polyethersulfone (PES), polytetrafluoroethylene (PTFE), and nylon 6,6. Each polymer interacts differently with various process fluids.
PES membranes offer broad pH compatibility and low protein binding, making them a common choice for biologics and protein-based formulations. Nylon offers broad solvent compatibility, although its pH range is more limited, particularly under acidic conditions. PTFE membranes provide excellent chemical resistance and are particularly suited for applications involving aggressive chemicals and solvents. PTFE is naturally hydrophobic, so membrane wetting requirements should be considered when selecting it for liquid filtration applications.
Critical Process Filtration manufactures sterilizing-grade filters using proprietary cast PES, PTFE, and nylon membranes, with each validated for bacterial retention per ASTM F838 protocols.
Relying solely on the stated pore size rating to predict filter performance can lead to problems. Multiple process variables influence how a filter actually retains microorganisms in your specific application. These variables include fluid chemistry, pH, viscosity, temperature, flow rate, differential pressure, and filtration time.
Research published in the PDA Journal of Pharmaceutical Science and Technology has documented cases where Brevundimonas diminuta and Ralstonia pickettii penetrated 0.2 and 0.22 micron rated membranes under certain process-specific conditions.
These findings reinforce that bacterial retention is not determined by the pore size number alone. Your validation protocol should include bacterial challenge testing with the actual process fluid under conditions that simulate or exceed your manufacturing parameters.
A bubble point test is a nondestructive integrity test used to verify that a membrane filter meets its specified retention characteristics. The test works by pressurizing one side of a wetted membrane with air and measuring the pressure at which the first steady stream of air bubbles passes through the largest pores.
The bubble point value correlates to the largest effective pore in the membrane. A higher bubble point indicates smaller maximum pore size and tighter retention.
Filter manufacturers establish a minimum bubble point specification for each membrane type, and every filter must meet or exceed that threshold. This test confirms that the filter's pore structure is intact and consistent with the validated retention performance.
Start by defining your critical process parameters: fluid composition, temperature range, required flow rate, batch volume, and allowable pressure drop. These factors determine which membrane polymer and pore size distribution will perform reliably in your process.
A filter rated at 0.2 micron from one manufacturer and a filter rated at 0.22 micron from another may both be suitable, provided each passes process-specific validation testing.
Consider requesting a filterability analysis before committing to a specific filter for production. This test evaluates how your process fluid interacts with the candidate filter membrane, measuring throughput, flow rate decay, and potential fouling behavior.
Critical Process Filtration offers testing and analytical services that help you identify the right filter configuration before you scale up to production.
The 0.2 versus 0.22 micron distinction is a difference in manufacturer labeling convention, not, by itself, a difference in bacterial retention. Both ratings identify sterilizing grade membranes validated to retain B. diminuta at the level required by ASTM F838.
Your filter selection decision should be driven by process-specific validation data, membrane polymer compatibility, and the operational conditions in your facility.
If you need support evaluating sterilizing filters for your application, Critical Process Filtration's Application Engineers can help you design a validation strategy and identify the right filter media for your process conditions. Let us know how we can help.
No. Both 0.2 and 0.22 micron filters are validated to retain Brevundimonas diminuta at 107 CFU/cm2, which is the standard for sterilizing grade filtration. The difference is in the manufacturer's rating convention, not in retention performance.
ASTM F838 is the standard test method for determining bacterial retention of membrane filters used in liquid filtration. It uses B. diminuta as the challenge organism and requires complete retention at a high challenge level to qualify a filter as sterilizing grade.
Yes. Critical Process Filtration manufactures sterilizing grade membrane filters in PES, PTFE, and nylon 6,6 media. Every filter is 100% integrity tested and validated for bacterial retention, meeting or exceeding ASTM F838 protocols.
Process fluid chemistry, temperature, flow rate, and filtration time all affect bacterial retention. A filter qualified under standard conditions may behave differently in your specific process. Critical Process Filtration's verification and validation services test filters with your actual process stream to confirm performance.
Common membrane polymers include polyethersulfone (PES), polyvinylidene fluoride (PVDF), and nylon 6,6. Each material offers different chemical compatibility and binding characteristics. Critical Process Filtration's Application Engineers can help you select the right membrane for your fluid and process requirements.