SOP: Sterility Testing by Membrane Filtration and Direct Inoculation (USP <71>)

This is a ready-to-use SOP for performing the compendial sterility test under USP <71> (harmonized with Ph. Eur. 2.6.1 and JP 4.06). Replace every <<FILL: ...>> placeholder, set your document numbers and dates, and route it through your normal document control, review, and approval. A worked filled specimen follows. The compendial chapters are copyrighted; this SOP describes the procedure in original wording and cites the chapters by number, and you must read the current chapter and validate the method for your product before use. This is general guidance to adapt and verify, not legal or regulatory advice.

Document control header

1. Purpose

This procedure defines how <<FILL: COMPANY NAME>> performs the end-product sterility test so a released sterile product is confirmed, under controlled aseptic conditions, to show no microbial growth in the compendial media over the incubation period. The test is a final confirmatory check; sterility is assured by the validated process, the contamination control strategy, environmental monitoring, media fills, and sterilization validation, not by this test alone.

2. Scope

This procedure applies to sterility testing of <<FILL: product types, e.g. terminally sterilized and aseptically filled injectables>> by membrane filtration or direct inoculation. Method suitability for each product is established under <<FILL: protocol ID for method suitability>> before this SOP is used to release that product.

3. Responsibilities

4. Definitions

• Membrane filtration: passing filterable product through a 0.45 micron membrane that retains micro-organisms, rinsing to remove inhibitory residue, then incubating the membrane in media.
• Direct inoculation (direct transfer): transferring product directly into media, used only when the product cannot be filtered.
• FTM: fluid thioglycollate medium, for anaerobes and aerobes, incubated at 30 to 35 C.
• SCDM (TSB): soybean-casein digest medium, for aerobes and fungi, incubated at 20 to 25 C.
• Method suitability (bacteriostasis and fungistasis): the prior demonstration that the product and test conditions do not inhibit growth of a low inoculum.

5. Prerequisites (verify before starting)

1. Method suitability is current for this product, strength, container, and method.
2. Every media lot in use has passed growth promotion and its own sterility check (see <<FILL: GPT log ID>>).
3. The Grade A / ISO 5 test zone (isolator or unidirectional-airflow cabinet in a controlled background) is qualified and within its monitoring limits.
4. Rinsing fluids, canisters, membranes, and any neutralizer or inactivating enzyme are the validated types for this product.
5. Samples are the correct number of units and quantity per container for the batch size and fill volume (per the USP <71> sampling tables and <<FILL: sampling determination worksheet>>).

6. Procedure

6.1 Set up and controls

1. Sanitize and set up the Grade A zone; start environmental monitoring for the session (viable air, settle plates, surfaces, personnel glove and gown; continuous non-viable counts in an isolator).
2. Prepare the mandatory controls: uninoculated media-only negative controls (FTM and SCDM), and a method negative control (rinse fluid and membrane processed without product).

6.2 Membrane filtration (preferred whenever the product is filterable)

1. Aseptically pool and filter the sampled product through the validated 0.45 micron canister system.
2. Rinse the membrane with the validated number of portions of the validated rinsing fluid (for example, Fluid A, or Fluid D for oily or hard-to-wet products), not exceeding the validated total rinse volume.
3. Add FTM to one canister and SCDM to the other (or transfer the membrane halves into the two media per the validated method).
4. Include any validated neutralizer or inactivating enzyme (for example, beta-lactamase for beta-lactam antibiotics) established in method suitability.

6.3 Direct inoculation (only when filtration is not possible)

1. Transfer the product directly into FTM and SCDM, keeping the product volume within the compendial ratio to the medium so the product does not itself inhibit growth.
2. Apply the validated dilution or neutralization for any antimicrobial product, as demonstrated in method suitability.

6.4 Incubate and read

1. Incubate FTM at 30 to 35 C and SCDM at 20 to 25 C for not less than 14 days.
2. Examine the media for macroscopic growth (turbidity) periodically and at the end of incubation.
3. Where the product turns the medium cloudy or makes reading difficult, follow the validated transfer step (transfer a portion to fresh medium on the defined day and incubate the transfer for the additional defined period).
4. Do not read positives early and stop, and do not extend incubation without a documented basis.

6.5 Read the controls

1. The media-only and method negative controls must show no growth. Growth in a negative control compromises the run and is handled under section 6.6 and the positive-investigation procedure.

6.6 Any growth

1. Treat any growth as a potential sterility failure. Do not discard the tube. Quarantine the batch and escalate to QA and the SME the same day.
2. Preserve the isolate for identification and follow <<FILL: positive-investigation record ID>>.

7. Acceptance criteria

• Negative controls show no growth for the full incubation.
• The product test shows no growth in either medium over not less than 14 days, read at the correct temperatures.
• All controls, environmental monitoring, and media growth promotion for the run are documented and within limits.
• Any growth is handled as a positive under the investigation procedure, never dispositioned informally.

8. References

21 CFR 211.167(a) (sterility testing of purported-sterile products); 211.165 (testing and release); 610.12 (biologics sterility, performance-based). USP <71> Sterility Tests; Ph. Eur. 2.6.1; JP 4.06 (harmonized via ICH Q4B Annex 8). USP <1211> Sterility Assurance; USP <1223> Validation of Alternative Microbiological Methods. FDA Guidance, Sterile Drug Products Produced by Aseptic Processing (2004); EU GMP Annex 1 (2022).

Related deliverables: Protocol: Sterility Test Method Suitability, Record: Sterility Test Positive Investigation and Invalidation Decision, and Log: Sterility Media Growth Promotion Test Record. Confirm every reference against the current published version before issue.

9. Records generated

• Sterility test worksheet (setup, method, rinse, controls, incubation reads, sign-offs).
• Environmental monitoring records for the test session.
• Positive-investigation record, where a positive occurs.

10. Revision history

11. Approvals

Filled specimen

A completed setup record for one batch, illustrative only.

Because method suitability had shown this product needs five rinses plus beta-lactamase to let a low inoculum grow, the analyst used exactly that validated method, ran all controls, incubated the full 14 days at the two temperatures, and released the result only after the negative controls read clean. That is the defensible shape of a compendial sterility test.

Common inspection findings this SOP prevents

• The test was run with a method not shown suitable for the product, so a no-growth result is not meaningful.
• Media were used before growth promotion results were available, or a lot was never tested.
• Both media were incubated at one temperature, or over-oxidized FTM was used.
• A positive was dispositioned informally instead of through a documented investigation.
• Negative controls or session environmental monitoring were missing, so a positive could not be adjudicated.

How to adapt this SOP

1. Set your document number, owner, and effective date, and name your validated canister system and rinsing fluids.
2. Point section 5 and section 6.6 to your real method-suitability protocol, GPT log, and positive-investigation record.
3. State your isolator or cabinet arrangement and its qualified background grade.
4. Reference your product-specific method sheets for rinse counts, neutralizers, and quantity per container.
5. Confirm the sampling tables and every reference against the current USP <71> before issue.