In critical process applications—such as pharmaceutical manufacturing, biotechnology, and high-purity chemical production—filter performance must remain consistent, reliable, and verifiable. Even when filters have been validated during development, regular integrity testing helps confirm that they continue to function as intended under actual process conditions.

This blog outlines key aspects of filter integrity testing—what it involves, when it’s performed, and the common methods used. It also looks at how integrity testing is applied in automated and single-use systems, where Sanitek continues to support manufacturers with advanced filtration solutions.

What Is a Filter Integrity Test?

A filter integrity test is a critical procedure used to verify that a membrane filter remains intact and is functioning according to its design specifications. Its primary purpose is to ensure that the filter can reliably retain bacteria and other microbial or particulate contaminants.

These tests are essential in regulated industries such as pharmaceuticals, food and beverage, and cosmetics, where sterile filtration is mandatory for product safety and regulatory compliance.

Rather than relying on direct microbial challenge tests, which are labour-intensive, time-consuming, and require complex laboratory protocols, industry best practices favour the use of validated physical testing methods.

These include techniques such as the bubble point test, diffusion test, and pressure hold test. Each of these methods provides quantifiable, non-destructive measurements that correlate with microbial retention, offering a practical and efficient alternative to biological testing.

Furthermore, these tests align with regulatory standards, including those set by the U.S. Food and Drug Administration (FDA) and other international quality bodies, ensuring consistent and defensible filtration performance.

Among the most important integrity testing strategies is the Post-Use Pre-Sterilisation Integrity Test (PUPSIT). This test is conducted after filter installation but before sterilisation, helping confirm that the filter is undamaged and properly seated.

Additionally, integrity tests are often performed both before and after use, particularly in critical applications, to verify the filter’s condition throughout its operational lifecycle. These multiple checkpoints significantly reduce the risk of contamination, verify system integrity, and support robust quality assurance protocols.

Why Validate Filtration Processes?

Validating filtration processes ensures that filters and the entire filtration system are functioning as designed. In industries such as pharmaceuticals, biotechnology, and food & beverage, process validation is key to meeting regulatory requirements and ensuring product safety. By validating filtration, manufacturers can guarantee that their system consistently performs as expected, minimising the risk of contamination or product failure.

It also helps confirm that the filtration process is effective in removing unwanted particles or microorganisms, ensuring compliance with FDA, ISO, or other industry-specific standards. Regular validation provides ongoing confidence that the system will continue to deliver high-quality results, reducing the likelihood of operational issues or regulatory violations.

Why Test the Integrity of a Filter That’s Already Been Validated?

Validation confirms a filter’s design capability, ensuring it meets defined performance standards under controlled conditions. However, real-world factors—such as handling during transport, installation errors, or variations in the manufacturing process—can affect filter performance after initial validation. These issues may lead to leaks, reduced retention efficiency, or increased risk of contamination.

Integrity testing helps detect any physical damage or changes that may occur after validation, confirming that filters continue to perform as expected under actual operating conditions. Beyond verifying performance, these tests also support GMP compliance, helping manufacturers meet regulatory requirements and avoid costly production disruptions.

When Should I Take The Integrity Test? H2

Integrity testing timing depends on your filtration objectives and process criticality. Here are key testing points for optimal quality assurance:

• Pre-Installation – Verify filters arrive undamaged from shipping
• Post-Installation – Confirm proper seating with no seal leaks
• Post-Sterilisation – Check after steam, chemical, or thermal treatment
• Pre-Process – Final validation before production begins
• Post-Process – Essential for critical sterile applications

For high-risk pharmaceutical/biotech operations, testing at multiple stages is recommended to maintain sterility assurance throughout the filtration lifecycle. The testing frequency should reflect your product requirements and regulatory standards.

Methods in Filter Integrity Testing and How They Work?

Several physical methods are used to verify the integrity of membrane filters, each with specific applications for hydrophobic and hydrophilic membranes. These tests help assess filter condition by evaluating characteristics such as pore size, airflow, pressure stability, and housing integrity. The most commonly applied methods include:

Bubble Point Test

The bubble point test is a standard method for verifying membrane filter integrity. Before testing, the filter is wetted to ensure all pores are filled with a thin liquid layer. Gas—typically air or nitrogen—is then introduced, and the pressure is gradually increased.

At a certain threshold, the gas displaces the liquid from the largest pores, causing a sharp rise in gas flow and visible bubble formation. This pressure, known as the bubble point, directly reflects the filter’s minimum pore size. If bubbling begins at a lower-than-expected pressure, it may indicate membrane damage or structural defects.

Diffusion Test

Also known as the forward flow test, the diffusion test measures the rate at which gas diffuses through a wetted membrane under constant pressure. It is commonly used for filters with larger surface areas, where the diffusion of gas is easier to measure.

The test provides valuable insights into membrane consistency and can help identify deviations that may affect the filter’s performance, including flow or retention capabilities.

Pressure Hold Test

The pressure hold test, also referred to as the pressure decay test, evaluates a filter’s ability to maintain pressure over a period of time. A stable pressure reading indicates that both the membrane and housing are intact, while any drop in pressure may signal leaks or compromised integrity.

This method is widely used due to its simplicity and effectiveness in detecting small defects, ensuring the filter’s operational reliability.

Water Intrusion Test

Used primarily for hydrophobic membrane filters like PTFE, the Water Intrusion Test (WIT) evaluates whether water enters the membrane under applied pressure, typically around 2.5 bar. The test is conducted with the filter in a dry state, without the need for pre-wetting agents.

During the test, any pressure drop caused by water diffusion is monitored over a fixed duration. A common acceptance range is 5–11 ml of water per 10″ filter within 10 minutes. Readings above this range may suggest damage to the membrane, improper installation, or a loss of hydrophobic properties.

How Do I Test My Filters for Integrity?

To test your filters for integrity, follow these essential steps:

1. Select the Appropriate Test Method – Choose between bubble point, diffusion flow, pressure hold, or water intrusion tests, depending on your membrane filter type (hydrophilic/hydrophobic) and specific application.

2. Prepare the Filter – Ensure the membrane is properly wetted (water for hydrophilic membranes, alcohol/water mix for hydrophobic ones). Install the filter correctly in its housing, ensuring seal integrity.

3. Connect to Test Equipment – Use either an automated integrity tester or a manual setup with calibrated pressure gauges/flow meters.

4. Run the Test – Follow the standard procedure for your chosen method, applying pressure gradually and recording the results.

5. Compare to Specifications – Verify the results against the manufacturer’s validated limits for pore size and material.

6. Document Results – Keep accurate records for quality control and regulatory compliance.

For critical applications like sterile filtration, perform tests both before and after use (PUPSIT). Automated testers improve accuracy by eliminating human error.

Filter Integrity Test: FAQS

What is a Filter Integrity Test?

A filter integrity test is conducted to verify and ensure that the membrane’s quality and performance comply with regulatory requirements. If a filter fails the test, it indicates that the filter is no longer usable, and any previous batches filtered by it may need to be inspected as well.

What is a Bubble Point Test?

The bubble point test relies on the principles of capillarity and surface tension. When a liquid flows through the filter membrane’s pores, it fills them due to surface tension and capillary action. Pressure is applied to expel the liquid from the pores, and the minimum pressure required to displace the liquid helps measure the pore size.

What is a Water Intrusion Test?

Primarily used for hydrophobic filters, the water intrusion test detects quality defects. These filters are typically used as vent filters. Water is applied under pressure from the upstream side of the filter, and the results are compared with the manufacturer’s specifications to assess filter integrity.

What is a Bubble Point?

The bubble point refers to the minimum pressure at which the first visible air bubble emerges from the filter pores. It is an essential parameter in determining the integrity and pore size of the filter.

What Can a 0.22 Micron Membrane Filter Remove?

A 0.22-micron membrane filter maintains consistent pore size, widely regarded as one of the smallest available. It is effective at removing fine particles, including bacteria, but is not capable of removing viruses.