Preservative efficacy testing evaluates whether a cosmetic formulation can control deliberately introduced microorganisms under defined laboratory conditions. Also called a challenge test, it helps manufacturers assess the effectiveness of a product’s antimicrobial protection.
ISO 11930 is a commonly used cosmetic-specific method. Testing generally runs over 28 days, with microbial counts assessed at specified intervals.
A passing result supports the product’s microbiological safety assessment. It does not replace good manufacturing practices, stability studies, or routine microbial quality testing.
What Is Preservative Efficacy Testing?
Preservative efficacy testing, often shortened to PET, challenges a cosmetic formulation with selected bacteria, yeast, and mold. The laboratory measures how those organisms survive or decline over time.
The test evaluates protection in the finished formulation. A preservative supplier’s technical data cannot establish that the same ingredient will perform effectively in every cream, cleanser, or serum.
Other ingredients, pH, manufacturing conditions, and interactions within the formula can affect performance. The important question is whether the complete product provides adequate protection under the selected test conditions.
Why Cosmetics Need Antimicrobial Protection
Cosmetics can encounter microorganisms through raw materials, water, equipment, filling operations, and consumer handling. Products used repeatedly may face additional exposure after opening.
For example, a facial cream in a jar may be touched with fingers throughout its use. A low microbial count immediately after manufacturing does not establish that its preservation system can control later contamination.
Testing therefore works alongside production controls. CGA’s ISO 22716 cosmetics GMP program addresses manufacturing practices such as hygiene, material handling, quality control, and documentation.
Which Products Need a Challenge Test?
Water-containing products commonly require careful preservation assessment. Examples include:
- Creams and lotions.
- Water-based serums and gels.
- Shampoos and conditioners.
- Liquid cleansers.
- Emulsions and other moisture-containing formulations.
However, not every cosmetic needs the same testing approach. ISO 29621 guides identifying microbiologically low-risk products through a documented risk assessment. Do not treat a product as exempt simply because it is described as “anhydrous,” “natural,” or “preservative-free.”
Consider formulation characteristics, packaging, intended use, and reasonably foreseeable exposure. Ask a qualified microbiologist or safety assessor to determine the appropriate approach.
How Does the Challenge Test Work?
1. Define the Method and Sample
Agree the method, acceptance criteria, sample quantity, and reporting requirements with the laboratory before submission.
ISO 11930:2019, including its applicable amendment, provides a reference approach for evaluating cosmetic antimicrobial protection. Its preservation efficacy test is primarily designed for water-soluble or water-miscible products; other matrices may require modifications.
Supply the final formulation and identify its batch, manufacturing date, pH, and preservative system.
2. Introduce the Test Organisms
The laboratory inoculates separate product portions with specified microorganisms. The standard panel includes Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans, and Aspergillus brasiliensis.
Additional organisms may be considered where product-specific risks justify them.
3. Measure Microbial Survival
The laboratory stores the challenged samples under defined conditions and measures surviving organisms at the required intervals. A typical study extends to day 28, with assessments at days 7, 14, and 28 as applicable to the organism and criteria.
Results are expressed through microbial counts and log reductions. A three-log reduction, for example, represents a thousandfold reduction from the starting count.
4. Confirm Method Suitability
The laboratory must demonstrate that its recovery procedure can detect surviving microorganisms. This includes neutralizing the product’s antimicrobial activity during enumeration.
Otherwise, preservative carried into the counting procedure could continue suppressing organisms and make the result appear better than it is.
What Do the Results Mean?
ISO 11930 includes acceptance criteria A and B. Interpretation depends on organism-specific reductions, subsequent growth, and the overall protection assessment.
Criterion B requires supporting risk assessment demonstrating additional protective factors beyond the formulation. Do not treat it as an interchangeable pass without further justification.
Ask for a report that identifies:
- The method and version used.
- Product and batch identification.
- Test organisms and starting counts.
- Results at each required interval.
- Method suitability findings.
- Applicable criteria and conclusion.
- Deviations or limitations affecting interpretation.
A failed result requires investigation. Review formulation, pH, preservative compatibility, manufacturing conditions, and test validity before deciding on corrective action.
Challenge Testing vs Other Cosmetic Tests
These assessments answer different questions.
| Assessment | Main purpose |
| Preservative efficacy test | Evaluates control of deliberately introduced microorganisms |
| Routine microbial quality test | Measures contamination present in the tested sample |
| Stability study | Evaluates changes under defined storage conditions |
| Packaging compatibility assessment | Examines interactions between the product and packaging |
A challenge test does not independently establish shelf life, packaging suitability, or overall product safety.
Likewise, GMP certification does not demonstrate that every formulation has passed a challenge test. CGA’s GMP audit services assess manufacturing systems and records; product-specific laboratory evidence serves a separate purpose.
When Should Manufacturers Reassess or Retest?
Evaluate whether existing results remain applicable when changes affect antimicrobial protection.
Relevant changes include:
- Preservative type or concentration.
- Formulation pH.
- Ingredients or supplier specifications.
- Manufacturing process.
- Packaging or dispensing format.
- Intended use or storage conditions.
Do not automatically extend one formula’s results to every fragrance, color, or product variant. Any grouping or reliance on existing evidence needs technical justification.
For example, changing a pump dispenser to a wide-mouth jar may alter contamination exposure. Review whether the previous assessment still supports the revised presentation.
What Records Should Manufacturers Maintain?
Keep the test report linked to the approved formulation, batch records, specifications, and change-control history. Document who reviewed the findings and any additional justification required.
Maintain clear responsibilities when outsourcing testing. Confirm laboratory competence, method suitability, and who will interpret the report.
CGA’s cosmetics GMP consulting guidance explains how manufacturers can organize procedures and records within a consistent quality system.
Strengthen Your Cosmetics Quality System With CGA
Challenge testing provides evidence about formulation protection. Reliable manufacturing requires that evidence to remain connected to supplier controls, production practices, and product changes.
Contact CGA to discuss cosmetics GMP readiness, documentation, and certification requirements.
Frequently Asked Questions (FAQs)
The challenge period typically lasts 28 days. Allow additional time for sample preparation, method suitability work, laboratory scheduling, and the final report.
Not necessarily. A documented assessment may identify a product as microbiologically low risk. The decision should consider formulation, packaging, intended use, and applicable requirements.
No. Microbial quality testing checks contamination present in a sample. Challenge testing evaluates how the formulation controls microorganisms deliberately introduced under defined conditions.
No. It supports the antimicrobial protection assessment under the tested conditions. Shelf-life justification also requires consideration of stability, packaging, and other relevant safety evidence.
Yes. A preservative-free claim does not establish microbiological safety. The product still needs an appropriate assessment of antimicrobial protection and contamination risk.
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