Preservative efficacy testing — often called a challenge test or PET — is one of the most important safety checks for any water-containing cosmetic. This article explains what it is and why it cannot be skipped.
In simple terms, a challenge test deliberately exposes your product to microorganisms and measures whether the preservative system kills them off. It answers a critical question: will this product stay safe from microbial growth in real-world use?
Why water means risk
Bacteria, yeast and mould need water to grow. That means any cosmetic containing water — lotions, creams, serums, cleansers, mists — is at risk of contamination, both during manufacture and every time a customer dips their fingers into the jar.
A preservative system keeps that growth in check. But preservatives are not one-size-fits-all, and the only way to know yours works is to run a preservative efficacy test on your finished formulation.
How a challenge test works
Known amounts of test organisms are added to your product, and samples are checked over time to see how quickly the preservative reduces them. A robust system knocks the population down fast and keeps it suppressed; a weak one lets it survive or rebound.
The result tells you clearly whether your product is adequately preserved or needs reformulating.
Test the finished product, not the data sheet
Preservative performance depends on the whole formula. pH, surfactants, proteins, clays and even packaging can all reduce a preservative's effectiveness. A preservative that performs perfectly in its raw-material data sheet can fail in your specific formulation.
That is why efficacy must be tested on your actual finished product in its actual packaging — exactly how we run it.
Preservation over shelf life
A preservative can also degrade over time, so passing once is not enough. Because of this, we include preservative efficacy assessment within our accelerated stability testing alongside the full thermal testing programme, giving you a fuller picture of safety across the product's life.
Frequently Asked Questions
Which products need a challenge test?
Any water-containing cosmetic, or one that can be contaminated with water in use, should be challenge tested. Truly anhydrous products such as pure oils and balms generally do not need one.
Is preservative testing included in your package?
Yes. Preservative efficacy assessment is built into our stability testing, and you can also order preservative efficacy (PET) testing on its own — get an instant quote for either option.