SterilityUSP <71> Sterility Tests

USP <71> is a compendial sterility test: a defined sample is inoculated into Fluid Thioglycollate Medium (FTM, for anaerobes and aerobes) and Soybean-Casein Digest Medium (SCDM / TSB, for fungi and aerobes), incubated at 30-35 °C and 20-25 °C respectively for 14 days, and observed for growth. A single growth-positive sample fails the test (subject to the specified retest provisions). The test detects viable, culturable organisms that grow in the specified media under the specified conditions.

What <71> does not provide: a guarantee of sterility (the sample size is too small — typically 20 units per test — to detect contamination below ~1 in 1000); detection of viable-but-non-culturable organisms; detection of organisms with growth requirements not met by FTM/SCDM. Sterility assurance comes from the upstream process (terminal sterilisation validation, aseptic process simulation, EM, CCIT, control of bioburden) — <71> is the final compendial check, not the primary source of assurance.

• Membrane filtration — preferred whenever the product can be filtered. Filter the entire test sample through a 0.45 µm membrane (or 0.22 µm for fine particulates), rinse to remove product inhibitory effects, then transfer the membrane (or sections) to FTM and SCDM. Used for solutions, suspensions that can be solubilised, oils, and products with inhibitory excipients.
• Direct inoculation — used when filtration is not feasible (insoluble suspensions, ointments, devices). The sample is inoculated directly into the media; volume-to-volume ratios are specified to ensure adequate dilution of inhibitory components.

Method suitability (sometimes called 'bacteriostasis/fungistasis testing') is required before either method can be used routinely — demonstrate that the chosen method recovers a small inoculum (≤100 CFU) of the USP-specified challenge organisms in the presence of the product. Failure of method suitability indicates product inhibition and requires method adjustment (more rinses, larger media volumes, neutralisation).

USP <71> specifies sample size by batch size (e.g. 20 units for batches of >500 units, fewer for smaller batches). With 20 units and the inherent ~1 in 1000 detection limit, the statistical conclusion the test supports is narrow: a passing <71> result is consistent with any contamination level below approximately 0.1-0.3 %. This is why <71> alone cannot demonstrate sterility — only that the upstream process is achieving a sterility assurance level (SAL) of typically 10⁻⁶ via validated terminal sterilisation, or that aseptic-process media fills are passing at the required level (typically <1 contaminated unit per 5,000-10,000 filled).

False-positive rates on <71> have historically been dominated by contamination introduced during the test itself rather than by genuine product contamination. Modern practice is to perform <71> in an isolator (USP <1208> describes isolator validation requirements), which essentially eliminates the operator as a contamination source and drives test false-positive rates to <0.1 % — versus 1-3 % for Grade A laminar-flow in Grade B cleanroom historical baselines. Annex 1 (2022) §10 strongly favours isolators for sterility testing.

ATP bioluminescence, solid-phase cytometry, flow cytometry, isothermal microcalorimetry and PCR-based methods can detect contamination in 24-72 hours rather than 14 days. USP <1223> describes the validation framework. RMM is increasingly used for cell/gene therapy products where 14-day sterility is incompatible with shelf-life; the regulator accepts RMM in lieu of <71> when the method is validated to equivalent or superior sensitivity, specificity and ruggedness. For traditional small-molecule and protein parenterals, <71> remains the routine method and RMM is added in parallel or for in-process testing.

1. Method suitability not re-verified after a formulation, container or filter change.
2. False-positive investigation that exonerates the sample without identifying the contamination source — regulators expect a documented, evidence-based exoneration before invoking the <71> 'invalid test' provision.
3. Sterility testing in Grade A LAF without isolator and no investigation of recurring environmental excursions on the test bench.
4. Reliance on <71> as the primary sterility-assurance evidence rather than on terminal sterilisation validation, media fills and EM.
5. Stability sterility pulls performed by <71> when validated CCIT-in-lieu-of-sterility is filed — duplicated effort that increases false-positive risk.
6. RMM deployed without the USP <1223> validation pack to support replacement of <71>.

• LIMS schedules <71> per the validated sampling plan; method suitability for each product/container combination is held as a controlled record with the date of last verification.
• Sterility test results are critical release attributes; a positive result triggers an immediate quarantine workflow, a documented investigation (with root-cause workflow), and recall-readiness if released stock could be impacted.
• Isolator and incubator data (cycle records, temperature, sanitisation) integrate with the test record so the auditor sees test environment and outcome together.
• CCIT-in-lieu-of-sterility substitution is documented per product; the stability schedule shows CCIT instead of <71> where qualified, with the supporting validation referenced.
• RMM, when used, is held alongside <71> with the validation pack reference and the per-result correlation visible.