Antimicrobial Effectiveness Testing: Oral Liquid Evidence Sheet

AET measures whether a product's preservative system stops added microorganisms from surviving or multiplying over 28 days. A pass shows the preservative works against five defined challenge organisms at a high inoculum. It does not show the product is sterile, and it does not predict every contamination event in use.

The United States Pharmacopeia chapter on the test, USP <51>, opens with the point that antimicrobial preservatives are substances added to aqueous pharmaceutical products. The test inoculates separate containers of the product with five organisms (strain numbers are from the American Type Culture Collection, ATCC), which are:

• Candida albicans (ATCC 10231)
• Aspergillus brasiliensis (ATCC 16404)
• Escherichia coli (ATCC 8739)
• Pseudomonas aeruginosa (ATCC 9027)
• Staphylococcus aureus (ATCC 6538)

The chapter specifies these strains and the starting levels. Older editions, such as the USP 32 text, name the mold Aspergillus niger. For Categories 1 to 3, the starting level is 10⁵ to 10⁶ colony-forming units per milliliter (cfu/mL), meaning living cells that grow into a visible colony on a plate. Counts are taken at days 7, 14 and 28, using a validated inactivator, called a neutralizer, in the plating medium.

Treat the result as a development tool. Robert R. Friedel of Perritt Laboratories wrote in 2000, about the USP 24 edition of the chapter, that the test should not be used as an absolute predictor of a product's ability to resist microbial contamination. Its job is to separate poorly preserved products from well-preserved ones before they reach patients.

An oral liquid with an aqueous base is Category 3 unless it is an antacid, which is Category 4. Category 3 needs at least a 1.0 log reduction in bacteria by day 14, and no increase at day 28. Yeast and mold must show no increase at days 14 and 28.

The criteria below are those of USP <51>, which USP-NF lists with an official date of 12 February 2025 as of October 2026. The full text is behind a USP-NF subscription, so check each row against your subscribed copy before writing it into a specification.

Category 4 starts from a lower inoculum, 10³ to 10⁴ cfu/mL. "No increase" is defined in the chapter as not more than 0.5 log₁₀ unit higher than the previous value measured. A syrup and a suspension are both Category 3 if the base is aqueous. For the formulation differences between them, see syrup vs suspension.

Reading a log reduction (our arithmetic). A 1.0 log reduction is a 90% kill, 2.0 log is 99% and 3.0 log is 99.9%. Take a Category 3 bacterial challenge starting at 1 × 10⁶ cfu/mL:

• At day 14 the count must be 1 × 10⁵ cfu/mL or lower.
• If it is exactly 1 × 10⁵, then day 28 may be at most 10⁵·⁵, about 3.2 × 10⁵ cfu/mL, because "no increase" allows 0.5 log above the previous value.
• If day 14 is 1 × 10⁴, the day 28 ceiling falls to about 3.2 × 10⁴. The allowance moves with the last reading.
• For a yeast or mold starting at 1 × 10⁶ cfu/mL, the reference is the initial count. Counts at day 14 and day 28 must each stay at or below about 3.2 × 10⁶ cfu/mL.

Yes. For products sold in Europe, the European Pharmacopoeia (Ph. Eur.) chapter 5.1.3 applies criteria A or B, set separately from USP <51>. Criteria A is the preferred standard and generally requires greater reductions than B. Criteria B is accepted only in justified cases where A cannot be attained, for example because of a greater risk of adverse reactions.

The chapter uses the same inoculum range, 10⁵ to 10⁶ microorganisms per milliliter or gram. It is reproduced in the 2012 British Pharmacopoeia appendix; take the current criteria for oral preparations from the edition of the European Pharmacopoeia in force.

If your product also ships to Europe, ask for the report to name whether criteria A or B was applied. The two chapters set their criteria separately, so a report written for one does not automatically cover the other. Decide at the start which markets the report must support.

AET is development and stability evidence. The International Council for Harmonisation's quality guideline on specifications, ICH Q6A, says antimicrobial preservative effectiveness should be demonstrated during development, during scale-up and throughout the shelf life, for example in stability testing. It adds that the attribute normally included in the specification is chemical testing for preservative content.

In practice this means:

• A batch certificate of analysis for a preserved oral liquid should list the preservative content, not a new 28-day challenge.
• The AET report should come from the development and stability programme, tied to the preservative level and the container the product ships in.
• If the formulation, preservative level, container or closure changes, ask whether the AET evidence still applies. That is our recommendation. The test results belong to a specific formulation and pack.

For the shelf-life side of this, see ICH stability zones, which sets the long-term storage conditions for your market.

It should require the category, the method suitability study, the full counts and the preservative level, each with a pass or fail you can check on receipt. Paste the table into your RFQ or quality agreement. Each row is a test you can run on the report when it arrives.

Row 4 is the one buyers skip most often. Without a validated neutralizer, carried-over preservative can stop organisms growing on the plate, so the count looks like a kill when it is only inhibition. USP <51> requires the neutralizer to be in the plate counts for this reason.

Record the test responsibilities in a pharmaceutical quality agreement. The third-party pharmaceutical manufacturing quote and vetting pack covers the rest of the supplier review.

One published example is a European laboratory that charges €784 per sample plus a €97 service fee per order for Ph. Eur. 5.1.3 testing, not USP <51> testing. It needs 200 g of sample and reports 7 to 8 weeks after receipt. These are the figures on that laboratory's own page as of October 2026, shown in euros and not converted.

Our arithmetic, assuming one €97 fee per order, from the page as of October 2026:

• One sample costs €784 + €97 = €881.
• Three samples in one order cost 3 × €784 + €97 = €2,449.

The 28-day incubation is a floor of 4 weeks. A turnaround of 7 to 8 weeks suggests the rest goes on sample receipt, method suitability work and reporting. That is our inference, not a figure the laboratory states. Plan the test early in development, because a failed result means reformulating and starting the 28 days again.

Quotes from US laboratories will differ. Ask each one what the price includes, in particular whether method suitability work is part of it or charged separately.

We are a technology-driven contract development and manufacturing organization (CDMO) for oral dosage forms, working from product development through commercial manufacturing. Oral liquids are among our finished-dose categories, and our oral liquid range lists 18 products.

Our buying route runs from inquiry to feasibility review, commercial quotation, development or technical transfer, production and quality control, packaging and dispatch. Put the evidence sheet above into your first inquiry, so the feasibility review starts from the same list you will check the report against.

Tell us the product, the dosage form, your target market and the preservative system you intend to use. We will respond with a commercial quotation. To reach us, email sunil@aarisepharma.com or message +91 9899783330 on WhatsApp, or use our contact page.