Methods of Sterilisation: General Introduction (Ph. Eur. 5.1.1)

In short: Ph. Eur. general chapter 5.1.1 sets out the principles behind sterilising medicines: sterility cannot be tested into a batch, so the process must be chosen carefully, validated and then followed exactly. Terminal sterilisation in the final container is preferred wherever the product can withstand it. This page is for QA, QC, microbiology and production staff and for pharmacy students.

What “sterile” means

A product is sterile when it contains no viable micro-organisms. In pharmacopoeial terms this is expressed as a sterility assurance level (SAL), the probability that a single unit is still non-sterile after the process. The chapter describes sterility as a SAL of 10^-6 or better, that is, no more than one chance in a million that a unit carries a surviving organism. EU GMP Annex 1 (2022) uses the same value in its glossary definition of terminal sterilisation.

Sterility is a critical quality attribute for:

• products that must be sterile because of how they are given, such as injections, eye preparations, intramammary products and some inhalation, irrigation and intrauterine products;
• products applied to badly injured skin, such as certain semi-solid preparations.

Why validation matters

Because micro-organisms are inactivated by an exponential process, there is always a very small, non-zero chance that one survives. For the same reason, the sterility of any single unit in a batch can neither be guaranteed nor proved by testing. Confidence comes from the process itself:

• study the effect of the chosen sterilisation method on the product and on its final container, so the product stays intact and effective;
• validate the process before it is used routinely;
• follow the validated process exactly. Any departure creates a risk of a non-sterile or degraded product.

EU GMP Annex 1 (revision published 2022) makes the same point: a manufacturer must not rely only on a terminal process or a finished-product test to assure sterility (paragraph 2.7).

What decides how well a process works

The effectiveness of sterilisation depends on:

• the method used and its conditions, such as time, temperature and moisture;
• the number and resistance of micro-organisms present before sterilisation (bioburden);
• the formulation of the product and the container it is in.

Choosing the method

1. Terminal sterilisation first. Sterilise the product in its final container wherever possible. Annex 1 (paragraph 8.34) says terminal sterilisation gives greater assurance of sterility than validated sterile filtration or aseptic processing.
2. Choose the container to suit the process. Where possible, pick a container and closure that allow the best sterilisation process for the product. The container system must keep the product sterile for its whole shelf life.
3. If terminal sterilisation is not possible, use sterile filtration through a bacteria-retentive filter and/or aseptic processing. Where feasible, add a further treatment of the filled container, such as heating, to improve sterility assurance. Annex 1 (paragraph 8.34) likewise suggests a terminal heat treatment combined with aseptic processing.
4. Prepare sterile products under suitable conditions and pack them in suitable containers. Annex 1 section 8 gives the cleanroom grades for preparation and filling.

Parametric release

When steam (moist heat), dry heat or ionising radiation is used as a fully validated terminal sterilisation process, a batch may be released on process data rather than on a sterility test of samples. This is allowed only with the approval of the competent authority. Where it is authorised, Annex 1 (paragraph 8.54) expects a robust system for lifecycle validation and routine monitoring, reviewed periodically, and refers to Annex 17 for further guidance on parametric release. Annex 17 limits parametric release to products sterilised in their final container (sections 4.1 and 4.3).

Related chapters

• Ph. Eur. 5.1.2 covers the use of biological indicators when validating sterilisation processes.
• Ph. Eur. 5.1.7 covers viral safety. For products of human or animal origin, or made using such material, validation must also show that relevant viruses are removed or inactivated.
• The methods in 5.1.1 apply mainly to bacteria, yeasts and moulds.

See also: Steam and dry heat sterilisation (Ph. Eur. 5.1.1, part 2), Production of Sterile Pharmaceutical Products and What is the meaning of sterility test?.

Frequently asked questions

Can sterility be proved by a sterility test?

No. A sterility test samples only a few units, so it cannot show that every unit is sterile. Assurance comes from a validated, controlled process, backed by monitoring.

Why is terminal sterilisation preferred over aseptic processing?

The product is sterilised in its sealed final container, which removes the contamination risks of handling after sterilisation. Annex 1 states that it gives greater assurance of sterility.

Is parametric release automatic once the process is validated?

No. It needs approval from the competent authority and a robust system of validation and routine monitoring.

How long must sterilisation records be kept?

Batch and sterilisation records must be kept at least 1 year after the expiry date of the respective product batch (EU GMP Part I Chapter 4, paragraph 4.11, or 5 years after QP certification if longer; 21 CFR 211.180(a)), and longer if local law or the marketing authorisation requires.

References

• European Directorate for the Quality of Medicines & HealthCare (EDQM), European Pharmacopoeia, general chapter 5.1.1 Methods of preparation of sterile products, with chapters 5.1.2 and 5.1.7 (current edition; full text is not freely available, so it is paraphrased here).
• European Commission, EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (revision published 2022), paragraphs 2.7, 8.34 and 8.54, and glossary.
• European Commission, EudraLex Volume 4, Annex 17: Real Time Release Testing and Parametric Release (Revision 1, 2018), section 4.
• European Commission, EudraLex Volume 4, EU GMP Part I, Chapter 4: Documentation, paragraph 4.11; US 21 CFR 211.180(a).