Sterility Testing Basics: USP <71> and Ph. Eur. 2.6.1

A sterility test checks whether viable microorganisms grow from samples of a batch labelled sterile, using two culture media incubated for at least 14 days. It is a pharmacopoeial release test, but it cannot prove a batch is sterile. It can only fail to find contamination in the units tested.

That is why regulators treat it as the last check in a wider sterility assurance system, alongside validated sterilisation or aseptic processing, environmental monitoring and media fills. Getting the method, environment and investigations right is what makes a pass meaningful.

Why a pass does not prove sterility

The test samples a small number of containers. If a batch has a low contamination rate, the chance of picking a contaminated unit is small. With 20 units tested, simple probability gives roughly:

• a 2% chance of detection if 0.1% of units are contaminated
• an 18% chance if 1% are contaminated
• an 88% chance if 10% are contaminated

So a sterility test detects gross failures, not low-level ones. Terminally sterilised products rely on a validated sterility assurance level, commonly 10⁻⁶, and aseptically filled products rely on process controls and media fill data. The test confirms; it does not assure.

Harmonised chapters, media and incubation

The sterility test is harmonised across USP <71>, Ph. Eur. 2.6.1 and the Japanese Pharmacopoeia. The chapters set the media, growth promotion, method suitability, number of containers by batch size, quantity per container, test techniques and interpretation. Check the pharmacopoeia required for each market. Two media are used together so that a broad range of organisms can grow:

• Fluid thioglycollate medium (FTM): primarily for anaerobic bacteria, also supports aerobes. Incubated at 30-35 °C.
• Soybean-casein digest medium (SCDM): for fungi and aerobic bacteria. Incubated at 20-25 °C.

Both are incubated for not less than 14 days and examined during and at the end of incubation. Each media lot must pass a growth promotion test with the specified compendial organisms, and media sterility must be shown by incubating uninoculated containers.

Rapid microbiological methods

Fourteen days is a long wait for short shelf-life products such as cell therapies. Rapid methods based on ATP bioluminescence, CO₂ detection or flow cytometry can shorten time to result. They must be validated as alternative methods, for example under USP <1223> or Ph. Eur. 5.1.6, and accepted by the regulator for each product.

Membrane filtration vs direct inoculation

Method suitability: bacteriostasis and fungistasis

Before routine use, each product must pass a method suitability test, sometimes called the bacteriostasis and fungistasis test. Media containing product, or membranes exposed to it, are inoculated with not more than 100 cfu of each specified organism. Growth must be visually comparable to controls without product.

If growth is suppressed, the method is modified, for example with more rinses, a neutraliser such as polysorbate or lecithin, or a larger media volume. Repeat suitability testing when the formulation, process or method changes.

Environment, controls and invalid tests

Sterility tests must be performed under aseptic conditions that do not themselves contaminate the samples. Many sites use an isolator, which greatly reduces the risk of false positives, or a Grade A unidirectional airflow cabinet in a Grade B background. The same monitoring principles apply as in cleanroom environmental monitoring.

Every session should include negative controls, such as sterile diluent or a known sterile article processed exactly like the samples, and environmental monitoring of the session. These are what you rely on when a positive appears.

If no growth is seen at the end of incubation, the product complies with the test. If growth is found, the product fails, unless the test can be shown to be invalid. The pharmacopoeias allow a test to be declared invalid only when one or more of these conditions is met:

1. Monitoring data of the sterility testing facility show a fault.
2. A review of the testing procedure reveals a fault.
3. Growth is found in the negative controls.
4. After identification, growth in the test can be unequivocally ascribed to faults in the materials or technique used.

If the test is invalid, it is repeated with the same number of units. If the repeat shows no growth, the product complies. If growth is found, it fails. Regulators expect strong evidence for invalidation, and a pattern of invalidated tests is itself a red flag.

Worked example: a positive FTM canister in a Dubai plant

A pharma plant in Dubai fills a preserved ophthalmic solution aseptically. Sterility testing uses closed membrane filtration in an isolator, with three rinses validated to remove the preservative. On day 9, one FTM canister becomes turbid. SCDM remains clear, and the negative control is clear.

1. Confirm and identify: a subculture and Gram stain show Gram-positive cocci, identified to species level as Staphylococcus epidermidis, a common skin organism.
2. Laboratory review: the post-session leak test of the isolator finds a pinhole in the right-hand glove, and the session glove print grew the same species.
3. Strain comparison: genotypic typing shows the test isolate and the glove print isolate match.
4. Manufacturing review: in parallel, QA reviews the batch record, filter integrity results, Grade A and B monitoring and the latest media fill. All are acceptable and the species was not recovered from filling-room monitoring.
5. Decision: the growth is unequivocally ascribed to a fault in the testing environment, so the test is declared invalid. The repeat test on the same number of units shows no growth, and the batch complies.
6. CAPA: glove change and pre-use integrity testing frequencies are reviewed, and glove checks are added before each session.

Had the strains not matched, or had filling-room data shown related organisms, the batch would have been rejected. Each invalidation should be reviewed through the site's OOS investigation process.

Common mistakes

• Treating a pass as proof of sterility rather than one element of sterility assurance.
• Skipping method suitability for a new strength or formulation change.
• Invalidating positives without strain-level evidence linking growth to the lab.
• Missing negative controls or session monitoring, leaving nothing to support an investigation.
• Using too few rinses for preserved or antibiotic products.
• Investigating the lab only, without a parallel manufacturing review.

Key takeaway: A sterility test can find gross contamination but cannot prove sterility. Its value depends on validated method suitability, a clean testing environment and rigorous, evidence-based investigation of every positive.

Frequently Asked Questions

How long is the incubation period for sterility testing?

USP <71> and Ph. Eur. 2.6.1 require incubation for not less than 14 days. FTM is incubated at 30-35 °C and SCDM at 20-25 °C. If the product makes the media turbid so growth cannot be judged, portions are transferred to fresh media and incubated further, as the chapters describe. Always follow the current pharmacopoeial text.

What is the difference between membrane filtration and direct inoculation?

In membrane filtration, the product is filtered and the membrane, which retains any organisms, is incubated in media. In direct inoculation, the product is added straight into the media. Membrane filtration is preferred where the product can be filtered, because rinsing removes antimicrobial components and larger volumes can be tested.

What is a bacteriostasis and fungistasis test?

It is the method suitability test that shows the product does not stop microorganisms growing under the test conditions. Media containing product, or membranes exposed to it, are inoculated with not more than 100 cfu of specified organisms. Growth must be comparable to controls without product before the method is used routinely.

Can a failed sterility test be repeated?

Only if the original test is shown to be invalid under the pharmacopoeial criteria, such as growth in negative controls or growth unequivocally linked to a laboratory fault. A repeat is not permitted simply because the first result was unexpected. If the test is valid and growth is confirmed, the batch fails the test.

Next steps

Our Pharmaceutical Microbiology & Contamination Control course covers sterility testing, method suitability, isolators and investigations for QC and QA professionals. For related reading, see OOS vs OOT results: how to investigate and cleanroom environmental monitoring: grades A to D. If you would like advice on where to start, speak to a counsellor.