Sterility Testing That Starts Before the Sample: Choosing the Right SCDM and Capsule Filters for USP <71> Compliance

How BD BBL™ Trypticase™ Soy Broth (Cat. 211768) and Whatman Cytiva Polycap™ Capsule Filters work together to give compounding pharmacies a defensible, reproducible sterility testing workflow — from media preparation through final filtration.

Most discussions of USP <71> sterility testing focus on what happens in the incubator: 14 days of observation, two media types, a pass/fail read at the end. But for compounding pharmacies serious about defensible results, sterility assurance actually begins much earlier — at the moment the culture medium is prepared and sterilized. The quality of media going into the test is just as critical as the conditions of the test itself.

This post examines how two product lines — BD BBL™ Trypticase™ Soy Broth (Catalog 211768) and the Whatman Cytiva Polycap™ capsule filter series — work in tandem across the sterility testing workflow: from reconstituting and sterilizing bulk SCDM media through to filtering compounded sterile preparations (CSPs) in compliance with USP <71> and USP <797>.

Why SCDM quality is the foundation of reliable sterility testing

Soybean Casein Digest Medium (SCDM), also called Tryptic Soy Broth (TSB), is one of two culture media mandated by USP <71> for sterility testing. Incubated at 20–25°C for 14 days, it detects aerobic bacteria and fungi — including all three of the reference organisms specified in the USP growth promotion test for SCDM: Aspergillus brasiliensis, Bacillus subtilis, and Candida albicans. Before any batch of SCDM is used in a sterility test, USP <71> requires that it pass a growth promotion test demonstrating visible microbial growth within the specified timeframe.

This means the medium itself must be nutritionally complete, correctly prepared at the right concentration, and sterilized without damaging its growth-promoting components. Substandard media — whether from inconsistent formulation, improper sterilization, or contamination during preparation — is a direct path to false-negative results, failed method suitability, and regulatory findings.

Product spotlight: BD BBL™ Trypticase™ Soy Broth

From powder to sterile medium: where capsule filters come in

Once BD Trypticase™ Soy Broth is reconstituted in purified water, it must be sterilized before use. Most compounding pharmacy laboratories sterilize liquid SCDM by autoclaving (15 minutes at 121°C) or by membrane filtration through a 0.22 µm capsule filter — the latter being preferred when heat-sensitive additives are present or when preparing smaller volumes for immediate use. This is precisely the first and critical application for the Whatman Cytiva Polycap™ capsule filter series: sterilizing the culture medium itself, before it ever touches a test sample.

A second role for capsule filters emerges during the USP <71> membrane filtration method: the compounded product being tested is passed through a sterilizing-grade capsule filter under aseptic conditions, trapping any microorganisms, which are then transferred to culture media for 14-day incubation. Choosing filters purpose-built for pharmaceutical filtration — with pre-sterilized, thermally fused, biosafe housings and documented integrity testing — is essential for both applications.

Product spotlight: Whatman Cytiva Polycap™ AS capsule filters

For biologics-sensitive applications: Polycap™ TC (PES) series

How the products fit the USP <71> workflow end-to-end

Understanding exactly where each product belongs in the process eliminates guesswork and supports a complete, auditable SOP.

Step 1 — Media preparation: Reconstitute BD BBL™ Trypticase™ Soy Broth (Cat. 211768) at 30 g/L in purified water. The dehydrated format allows batch-controlled preparation of precisely the volume required for your testing schedule, minimizing waste and media hold-time concerns.

Step 2 — Media sterilization by filtration: Pass the reconstituted SCDM through a pre-sterilized Polycap AS 36 (Cat. 6705-3602 or 6706-3602) at 0.2 µm using a peristaltic pump or pressure source. The GMF prefilter removes particulates from the broth, extending membrane life and preventing premature clogging. Use Cat. 6706-3602 (with filling bell) when dispensing directly into open media bottles to maintain asepsis at the point of transfer. For protein-enriched or biologically sensitive SCDM variants, the Polycap TC (Cat. 6715-3682) is the superior choice, given its low-protein-binding PES membrane and LAL-tested endotoxin performance.

Step 3 — Growth promotion verification: Inoculate a portion of sterilized BD Trypticase™ Soy Broth with the USP <71> reference organisms (Aspergillus brasiliensis, Bacillus subtilis, Candida albicans) and confirm visible growth within the specified incubation periods. Document lot numbers, preparation date, sterilization method, and results in your growth promotion records.

Step 4 — USP <71> sterility testing: For membrane filtration sterility tests, pass the CSP through a validated sterilizing-grade capsule filter (0.45 µm nominal). Rinse the membrane at least three times with validated sterile flush fluid, then add the sterilized BD Trypticase™ Soy Broth and Fluid Thioglycollate Medium for 14-day incubation — SCDM at 20–25°C, FTM at 30–35°C.

A note on method suitability and filter validation

USP <71> requires that the sterility test method be validated for suitability against each product formulation. For compounding pharmacies using membrane filtration, this validation includes confirming that the capsule filter does not retain or destroy the USP reference organisms at levels that would compromise test sensitivity. The Polycap TC series, with its documented HIMA challenge test performance and 1% per-unit integrity testing, provides a strong starting point for this validation documentation. Any change in filter type, membrane material, or pore size requires re-validation of method suitability before continued use in sterility testing.

Similarly, any change in the SCDM supplier or lot that results in altered growth promotion performance must be investigated and documented before that medium is used in a sterility test. The consistent, harmonized formulation of BD Cat. 211768 — supported by BD’s own lot release QC — reduces the likelihood of inter-lot variability and simplifies your growth promotion records.