Suitability of product-specific methods for complex matrices

Why standard methods alone are not enough

Herbal medicines and cosmetics are among the most challenging groups of products when it comes to microbiological quality control. In many cases, it is not the standardised method itself that poses the challenge, but rather the product-related matrix. Plant-based ingredients, such as essential oils, tannins, or alkaloids, can inhibit the growth of test organisms, and preservatives or aromatic substances in cosmetic formulations have been found to have similar effects. If these specific matrix effects are not taken into account to an adequate extent, there is a greater risk of false negative findings — i.e. results that appear to be normal, in spite of the fact that the detection capability of the method is limited.

Regulatory framework in the Ph. Eur. and ISO standards

Ph. Eur. 2.6.12 (determination of total viable aerobic count) and Ph. Eur. 2.6.13 (detection of specified microorganisms) both call for product-specific suitability testing, frequently referred to as product-related verification, for non-sterile medicinal products. The aim of this is to demonstrate that the method works reliably for the product concerned under the selected test conditions.

Particular attention is given to testing whether the growth of microorganisms is inhibited by constituents in the matrix. Appropriate measures, such as neutralisation, dilution, or adapted sample preparation, are defined as and when necessary. The evaluation is based on defined acceptance criteria, which usually refer to observed recovery rates following targeted inoculation.

A similar conceptual approach is adopted for cosmetics, whereby an evaluation of possible matrix effects also constitutes an integral part of the methods described in microbiological ISO standards, such as ISO 21149 (enumeration of aerobic mesophilic bacteria) and ISO 16212 (enumeration of yeast and mould).

Organism-specific standards, such as ISO 22718 (detection of Staphylococcus aureus), and overarching approaches, such as those described in ISO 18415 (detection of specified and non-specified microorganisms), are applied for specified microorganisms. Here, too, the focus is on monitoring matrix-related inhibition.

Method suitability testing vs. method verification

An important distinction must be made between method verification in accordance with ISO/IEC 17025 and verification of a method’s suitability for a specific product.

According to ISO/IEC 17025, the verification process shows that a laboratory is capable of implementing, mastering, and reproducibly performing a standardised method correctly. Product-specific suitability testing, on the other hand, provides an answer to the question of whether a method works reliably for the product in question.

The two verification concepts are therefore complementary – they each comply with different regulatory requirements but cannot be used as substitutes for one another.

Why inadequate suitability testing is such a critical issue

The informative value of microbiological test results is limited without reliable evidence of a method’s suitability. There is a risk that contamination will not be detected, particularly when examining antimicrobial botanical matrices or preserved formulations.

Approval decisions can only be scientifically substantiated to a limited extent in such cases. At the same time, the amount of effort and expense involved increases substantially if it is necessary to clarify suitability issues retrospectively – if neutralisation strategies or test conditions have to be subsequently adapted and documented, for instance.

Support for product-specific method suitability testing

Suitability testing of microbiological methods can be particularly time-consuming where complex botanical or cosmetic matrices are concerned.

PhytoLab provides support for manufacturers with respect to the planning and implementation of such suitability tests – from the development of an appropriate test concept, right through to the creation of an auditable suitability test report.

The aspects that are taken into consideration include the following:

• Definition of suitable neutralisation strategies
• Establishment of acceptance criteria and recovery rate assessment
• Scientifically verifiable documentation for authorities and auditors
• Integration of results into existing QM systems

Suitability tests may prove to be particularly worthwhile when launching new products, changing recipes, changing suppliers, transferring methods or expanding product portfolios.

A matrix group concept may also be an expedient solution for extensive product portfolios, whereby similar products can be grouped together and covered by a representative worst-case product, as long as the regulatory comparability of the matrices can be substantiated.

Conclusion

As far as medicinal products and cosmetics are concerned, providing evidence of product-specific method suitability is not an optional extra, but a prerequisite for reliable microbiological test results. The crucial factor is a test concept that takes possible matrix effects into account systematically, with results that are documented in a traceable and auditable manner.