The USP <71> sterility test is the microbiological assay used to verify the absence of viable microorganisms in a batch of sterile product: it is described in general chapter USP <71> of the United States Pharmacopeia and in chapter 2.6.1 of the European Pharmacopoeia (Ph. Eur.), two texts harmonised through the Pharmacopoeial Discussion Group (PDG). On paper it is a simple test — inoculate, incubate, observe — but in practice it is one of the most delicate assays in the QC laboratory: a false positive blocks a batch and triggers a complex investigation, while an execution that departs from the pharmacopoeia is an immediate audit finding. This guide covers methods, media, sample size and the handling of results, with the points QA and QC must keep under control to stay audit-ready.
USP <71> sterility tests and Ph. Eur. 2.6.1: what the pharmacopoeias require
The pharmacopoeial sterility test applies to substances, preparations and articles that are required to be sterile: parenterals, ophthalmics and any other dosage form for which the monograph requires it. Both chapters make a key point explicit: a satisfactory result only indicates that no contamination was found in the sample examined, under the conditions of the test. Sterility cannot be "tested into" the product — it is built in by the process. EU GMP Annex 1 (rev. 2022) reinforces this at paragraph 2.7, warning that sole reliance for sterility should not be placed on any terminal process or finished product test. The test nevertheless remains a release requirement: it must be performed under controlled aseptic conditions — in a laminar airflow cabinet within a cleanroom or, increasingly, in an isolator — that must not affect the outcome of the assay.
Membrane filtration or direct inoculation: choosing the method
The pharmacopoeias provide two techniques:
- Membrane filtration: the product is filtered through a membrane with a nominal pore size not greater than 0.45 µm, which retains microorganisms; the membrane is then rinsed (typically with at least three portions of rinsing fluid, to remove antimicrobial residues) and transferred to or incubated with the culture media. It is the technique of choice whenever the nature of the product permits: aqueous solutions, filterable alcoholic or oily preparations, soluble solids.
- Direct inoculation: a defined quantity of product is transferred directly into the medium, with the product volume not exceeding 10% of the volume of the medium. It is used when filtration is not practicable (suspensions, ointments that cannot be solubilised, particular devices).
The choice is not free: it must be justified by the nature of the product and supported by the method suitability test. In an audit, direct inoculation of a filterable product without a documented rationale is a classic observation.
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Culture media, temperatures and incubation times
The test uses two complementary media, each with its own incubation window. Inoculated media are incubated for not less than 14 days, with periodic observation and a documented final reading.
| Parameter | Fluid Thioglycollate Medium (FTM) | Soybean–Casein Digest (TSB/SCDM) |
|---|---|---|
| Target microorganisms | Anaerobic (and aerobic) bacteria | Fungi and aerobic bacteria |
| Incubation temperature | 30–35 °C | 20–25 °C |
| Minimum duration | 14 days | 14 days |
| Growth promotion (examples) | S. aureus, P. aeruginosa, C. sporogenes | C. albicans, A. brasiliensis, B. subtilis |
Each batch of medium must pass the sterility check of the medium itself and the growth promotion test: an inoculum of not more than 100 CFU per strain, with visible growth within 3 days for bacteria and 5 days for fungi. Unqualified media invalidate everything inoculated into them.
Method suitability: the prerequisite inspectors check first
Before applying the test to a product, you must demonstrate that the product itself does not inhibit microbial growth under the conditions of the assay (bacteriostasis/fungistasis). The method suitability test is performed by inoculating the reference strains (not more than 100 CFU) in the presence of the product, using the same technique as the routine test: if growth is comparable to the control without product, the method is suitable; if not, the conditions are modified — more membrane rinses, neutralisers, greater dilution — and the verification is repeated. Suitability must be re-assessed when the formulation, the method or a critical supplier changes. An incomplete or never-updated suitability file is among the most frequent findings in microbiology laboratory inspections.
Sample size: minimum quantities and number of containers
The pharmacopoeias define in harmonised tables both the minimum quantity to be tested per container and the minimum number of containers per batch:
- Quantity per container (Ph. Eur. Table 2.6.1.-2): for example, for liquids of 1 to 40 mL, half the contents of each container and in any case not less than 1 mL; for solids of 50 to 300 mg, half the contents and not less than 50 mg.
- Number of containers (Ph. Eur. Table 2.6.1.-3): depending on batch size; for parenteral preparations typically up to 20 containers per batch, for ophthalmics up to 10.
Sampling must be representative: for autoclaved products, include the coolest positions of the load; for aseptic processes, Annex 1 expects samples to include units filled at the beginning and end of the fill and after significant interventions. A "random" sampling plan without this rationale is indefensible in an inspection.
Positive result: interpretation, invalidation and retest
If after 14 days no medium shows growth, the product passes the test. If growth appears, the batch does not comply — unless the laboratory investigation demonstrates that the test is invalid. The pharmacopoeias allow invalidation only for unequivocal causes, for example:
- microbiological monitoring of the testing area reveals a fault or environmental contamination;
- review of the procedure identifies a technical error in the execution;
- the negative controls show growth;
- the isolated microorganism is unequivocally attributed to materials or technique unrelated to the product.
Only in these cases may the test be repeated with the same number of units; the batch complies if the retest shows no growth. Identification of the contaminant and comparison with environmental monitoring data are therefore an integral part of the OOS investigation, to be managed with the same documentation discipline as a critical deviation.
GuideGxP recommendation
Our operational advice is to treat the sterility test as a process, not a single assay: qualification of the testing environment (isolator or cleanroom), batch-by-batch media qualification, up-to-date method suitability for every product, a sampling plan with a written rationale and a predefined investigation flow for positives, linked to environmental monitoring. If the product is intended for several markets, verify the residual differences between Ph. Eur., USP and JP before an auditor does: even for harmonised chapters, editorial details and national monographs can diverge. On this point, our guide Pharmacopoeias Compared in GMP: managing USP – Ph. Eur. – JP – BP divergences provides ready-to-use matrices to document compendial decisions and defend them in audits.
