Standard Operating Procedure (SOP)

Here’s a detailed Standard Operating Procedure (SOP) for the Bubble Point Test, which is commonly used to verify the integrity of membrane filters—especially in sterile pharmaceutical or biotech processes.

1. Purpose:

To describe the standard method for performing the Bubble Point Test to verify the integrity of membrane filters used in sterile filtration.

2. Scope:

Applicable to all hydrophilic membrane filters used in sterile filtration processes within the facility, such as final product filtration, gas filters (if wetted), and critical process points.

3. Responsibilities:

• Quality Control / Quality Assurance: Review and approve test results.

• Production / Process Operator: Perform the bubble point test.

• Validation / Engineering: Maintain test equipment and train personnel.

4. Definitions:

• Bubble Point: The minimum pressure required to force air through a wetted membrane, indicating the largest pore size.

• Wetting Fluid: Typically Water or 60–80% IPA depending on membrane material.

5. Safety Precautions:

• Use appropriate PPE (gloves, lab coat, eye protection).

• Handle pressurized equipment with care.

• Ensure filters are properly clamped and secure before testing.

6. Materials and Equipment:

• Filter to be tested (pre-wetted)

• Clean compressed air or nitrogen supply

• Pressure regulator and gauge

• Water or IPA (for wetting)

• Bubble point tester or setup (manual or automated)

7. Procedure:

7.1. Pre-Test Checks:

1. Verify the filter part number, integrity specification, and expiry.

2. Ensure the filter is installed in a clean and secure test setup.

3. Wet the filter with appropriate fluid (e.g., water for PES, IPA-water mix for PTFE).

4. Remove all entrapped air by flushing the filter gently.

7.2. Performing the Bubble Point Test:

1. Gradually apply air or nitrogen pressure to the upstream side of the wetted filter.

2. Monitor pressure gauge while observing the downstream side (visually or using automated sensor).

3. Identify the pressure at which a continuous stream of bubbles appears from the largest pore — this is the Bubble Point.

4. Compare measured value to the manufacturer’s minimum acceptable value.

8. Acceptance Criteria:

• The observed bubble point must be equal to or greater than the validated minimum value provided by the filter manufacturer.

• If the test fails, the filter must be discarded or re-tested after re-wetting.

9. Post-Test Procedure:

• Document test results with date, time, operator, and equipment ID.

• Release or reject the filter based on test results.

• Clean the test setup as per SOP.

• Report any deviations or abnormalities.

10. Documentation:

• Record the following in the logbook or electronic system:

  • Filter ID / Batch No.

  • Test fluid used

  • Test pressure and result

  • Operator initials and date

  • Equipment ID

  • Remarks (if any)

11. References:

• Manufacturer’s filter validation guide

• USP <797> or <1223> (if applicable)

• ISO 13408 or equivalent

• Internal quality policies and validation protocols.

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