Preservative Efficacy Test

A preservative efficacy test, often called a challenge test, is a laboratory procedure in which a finished product is deliberately inoculated with bacteria, yeast and mould, then sampled over several weeks to see whether the organisms die back at the expected rate.

It exists because a preservative's performance cannot be predicted from the ingredient list. The same preservative behaves differently depending on pH, on what else is in the formula, on packaging and on how much water the product contains. A formula can pass with a preservative many people would call harsh and fail with the same one at a different pH.

• Passing is a condition of putting a water-containing cosmetic on the market in the EU and many other regions.
• The test underpins the period-after-opening figure printed on the pack.
• It tells you nothing about how a product performs on skin, only that it should not spoil under normal use.

Nothing on a label reports the result, so this is context rather than something a reader can check. Its practical value is in what it implies: a product marketed as preservative-free still has to survive this, which is why such formulas usually rely on alternative systems, low water content or restrictive packaging rather than on nothing at all.