You have spent months perfecting a cream, a serum or a shampoo. It looks great and smells even better. But before it can go on sale in the EU, one question decides whether it is safe to use for its whole shelf life: will the formula protect itself once the consumer opens the jar and starts dipping fingers into it? A cosmetic challenge test answers exactly that.
Contents
- What is a cosmetic challenge test?
- Which microorganisms are used?
- How does the test work?
- Ph. Eur. 5.1.3 vs ISO 11930: which standard applies?
- Is a challenge test required by law?
- Which products need one?
- What if my product fails?
- Challenge test vs microbiological specifications test
- Natural and organic formulas: a special case
- How to prepare for your challenge test
- Common mistakes to avoid
- When should you repeat the test?
- Frequently asked questions
- Key takeaways
What is a cosmetic challenge test?
A challenge test, also called a preservative efficacy test (PET) or antimicrobial effectiveness test, checks whether the preservation system in your finished product can keep microbes under control. In the lab, a sample of your product is deliberately contaminated with a known amount of selected microorganisms. Technicians then measure how quickly those microbes die off or stop growing over the following weeks.
The idea is simple: real life is not sterile. Hands, water, air and spatulas all introduce bacteria, yeasts and moulds into a product. A formula that passes the test has shown it can survive that kind of everyday abuse.
Which microorganisms are used?
Both common standards use a panel of five reference strains that represent typical contaminants:
- Pseudomonas aeruginosa – a Gram-negative bacterium that thrives in water-based products
- Staphylococcus aureus – a Gram-positive bacterium found on human skin
- Escherichia coli – an indicator of hygiene problems
- Candida albicans – a yeast
- Aspergillus brasiliensis – a mould
Read also Preservative Boosters and Multifunctionals: Can They Pass a Challenge Test?
How does the test work?
- Inoculation. Each microorganism is added to a separate portion of your product at a high starting concentration.
- Storage. The samples are kept at room temperature, protected from light, for 28 days.
- Sampling. At fixed time points (for example after 7, 14 and 28 days) the lab counts how many viable microbes are left.
- Evaluation. The reduction in microbial count, expressed in log steps, is compared with the acceptance criteria of the standard.
Because of the 28-day incubation, plan for roughly a month plus reporting time. This is one of the main reasons to order the test early in your launch schedule rather than the week before printing labels.
Ph. Eur. 5.1.3 vs ISO 11930: which standard applies?
Two methods dominate the market. Ph. Eur. 5.1.3 (Efficacy of Antimicrobial Preservation) comes from the European Pharmacopoeia and was originally written for pharmaceutical products. ISO 11930 is written specifically for cosmetics and is the reference most cosmetic safety assessors expect to see. Both use the same five strains and a 28-day timeline, and both define two sets of criteria:
- Criteria A describe the recommended level of efficacy.
- Criteria B are a lower threshold, accepted only when criteria A cannot be reached for justified reasons.
ISO 11930 also links the test to a microbiological risk assessment under ISO 29621, which lets you identify products that are so low-risk that a challenge test is not needed.
Is a challenge test required by law?
The EU Cosmetics Regulation (EC) No 1223/2009 does not name the test explicitly. It does, however, require every product to be safe under normal and reasonably foreseeable use, and the Cosmetic Product Safety Report (CPSR) must cover the microbiological quality of the product. In practice, safety assessors expect preservation efficacy data for any product that can support microbial growth. Without it, a CPSR is hard to sign off. The SCCS Notes of Guidance also treat microbiological quality as a core part of the safety evaluation.
Read also Challenge Test Cost: What Affects the Price
Which products need one?
You will almost certainly need a challenge test if your product:
- contains water, or is an emulsion (creams, lotions, serums, shampoos, shower gels)
- is packed in a jar or any container the consumer opens repeatedly
- contains plant extracts, hydrolysed proteins, sugars or other “food” for microbes
- is a “natural” or “preservative-free” product relying on alternative preservation
Products with very low water activity, high alcohol content or extreme pH are often classed as low-risk under ISO 29621, for example pure oils, balms and some alcohol-based sprays. Even then, the decision should be documented by a qualified assessor rather than assumed.
What if my product fails?
A failed challenge test is common, especially with natural formulas, and it is fixable. The usual levers are:
- raising the preservative concentration, within the limits of Annex V of the Cosmetics Regulation
- combining preservatives or adding boosters such as chelating agents
- adjusting pH, which strongly affects how well many preservatives work
- reviewing ingredients that feed microbes, and the packaging type
After every change, the test must be repeated on the new formula. That is why it pays to run it on a near-final recipe, before you commit to bottles and print runs.
Challenge test vs microbiological specifications test
The two are often confused. A microbiological specifications test counts the microbes in a sample as it is and checks for specified pathogens. It tells you whether a batch is clean today. The challenge test asks a different question: will the product stay safe tomorrow, after the consumer contaminates it? Most brands need both, plus a CPSR that brings the results together.
Read also My Product Failed the Challenge Test: What to Do Next
Natural and organic formulas: a special case
Natural cosmetics are the products that most often struggle with preservation. Plant oils, botanical extracts, honey, aloe and glycerin-rich bases are all excellent food for microbes, while the preservatives that are considered “natural” are usually weaker or work only in a narrow pH window. Brands that avoid classic preservatives therefore need to test earlier and more often, and to be ready for a few formulation rounds. Remember that “natural” does not mean “exempt”: the same legal safety duty applies, and the same challenge test is the proof.
How to prepare for your challenge test
A few simple steps save time and avoid inconclusive results:
- Use the final formula. Test the exact recipe you intend to sell, including the final preservative level, fragrance and colour. Even a small change can shift the result.
- Send the finished packaging. Ideally the product reaches the lab in its real primary pack, because the container, pump or dropper affects how it behaves in use.
- Send enough product. The lab needs a sufficient quantity for five separate inoculations and sampling at every time point.
- Know your formula. Give the lab the full ingredient list and the preservative system so that neutralisers can be chosen correctly, otherwise the preservative itself can mask the microbes during counting.
- Keep the batch record. Note the batch number and manufacturing date so that the report can be linked to a specific, traceable production batch.
Common mistakes to avoid
- Testing too late. A failed result discovered after the packaging is printed means expensive delays.
- Testing a lab sample, not a production batch. A small hand-made batch can behave differently from one made at scale.
- Forgetting variants. A new fragrance, a new colour or a different pack size may need its own assessment, or at least a documented justification.
- Treating a pass as permanent. The result only applies to the formula and packaging that were tested.
- Relying on supplier claims alone. A preservative supplier may state a recommended dose, but only a test on your own finished product shows that your system really works.
When should you repeat the test?
Run a new challenge test whenever something that affects preservation changes: a new formula, a different preservative or supplier, a new pH target, or different packaging. Keeping the report in your Product Information File also protects you if an authority asks for proof.
Frequently asked questions
Can I skip the challenge test if I use a lot of preservative?
No. A high dose does not prove efficacy, and it can also create safety and stability problems. The test shows that the amount you use is enough, and it helps you avoid using more than necessary.
Does a small batch or handmade product need a challenge test?
Yes, if it can support microbial growth. The legal duty to place a safe product on the market does not depend on the size of the business or the batch.
Is one report valid for the whole product line?
Only if the products share the same base formula, preservative system and packaging type, and a safety assessor can justify it. Otherwise each variant needs its own data.
Key takeaways
- A challenge test proves your preservation system works against realistic contamination.
- It takes 28 days of incubation, so schedule it early.
- Choose ISO 11930 or Ph. Eur. 5.1.3 and aim for criteria A.
- Most water-containing products need one; truly low-risk products may be exempt after assessment.
- The result feeds directly into your CPSR.