Foothill Ranch, CA
ACTA runs the USP <51> Antimicrobial Effectiveness Test under cGMP, challenging your preservative system with the five compendial organisms and measuring survivor reduction across the full 28-day study. Send us the product and a microbiologist scopes the work.
ACTA performs the USP <51> Antimicrobial Effectiveness Test, also called the preservative efficacy test (PET) or preservative challenge, under cGMP. The study confirms that the preservative system in a multi-dose product controls microorganisms introduced during normal use. It applies to multi-dose sterile products, non-sterile aqueous formulations, topicals, oral liquids, and similar preserved products.
Our microbiology laboratory runs this work in-house alongside related antimicrobial chapters. You receive a cGMP Certificate of Analysis (COA) with a log-reduction table for every organism at every timepoint, measured against the acceptance criteria for your product category.
USP <51> challenges the product with a defined panel of three bacteria and two fungi. Each organism is inoculated separately at roughly 1 x 10⁵ to 1 x 10⁶ CFU per mL, so the preservative has to control the range of microorganisms a product realistically encounters.
In an antimicrobial effectiveness test, each of the five organisms is inoculated into a separate portion of the product at roughly 1 x 10⁵ to 1 x 10⁶ CFU per mL. The inoculated product is held at 22.5 °C ± 2.5 °C and sampled at set timepoints, typically Day 7, Day 14, and Day 28. At each pull the surviving organisms are enumerated and the reduction from the starting count is reported as a log₁₀ value.
Before the study runs, method suitability is established under USP <51>. The preservative is neutralized so that any survivors are recovered accurately and the count is not artificially suppressed. Without that step, a product can look effective simply because the plating method carried preservative through and killed the organisms it was meant to count.
The 28-day timeline is inherent to the method. This is a real-time challenge, not something a laboratory can shorten, so plan for roughly six weeks from submission to final report. We tell you that up front so the schedule is clear before you commit.
Each organism is inoculated into the product, held at 22.5 C, and pulled at Day 7, 14, and 28. Survivors are counted, the log reduction is calculated, and the result is compared against the USP 51 criteria for your category. Hover any stage to see what it does.
USP <51> sorts products into four categories, and the category sets the log-reduction a preservative has to achieve. Injectable and other sterile products carry the most stringent criteria.
| Product category | Bacteria | Yeast and mold |
|---|---|---|
| Category 1 Injections, sterile | Not less than 1 log₁₀ reduction by Day 7, not less than 3 log₁₀ by Day 14, and no increase at Day 28. | No increase from the initial count through Day 28. |
| Category 2 Topical | Not less than 2 log₁₀ reduction by Day 14, and no increase from that point through Day 28. | No increase from the initial count through Day 28. |
| Categories 3 and 4 Oral, antacids | Each carries its own compendial log-reduction criteria under USP <51>. | No increase from the initial count through Day 28. |
A single preservative challenge on one formulation often sits behind a long queue at a large laboratory. ACTA takes the one product, establishes neutralization, runs the full 28-day challenge, and a microbiologist follows up on the result. Many of our largest accounts started as one small study.
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