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Standardizing Preservative Efficacy Test Data
Preservative efficacy testing is performed to determine the minimum effective concentration of anti-microbial preservatives required for adequate preservation of cosmetic and pharmaceutical products. Products must be preserved to prevent product spoilage and the growth of microorganisms that could render the products harmful to users.
Several methods of preservative efficacy testing are in use, including the United States Pharmacopeial Convention (USP) method, the Cosmetic, Toiletry & Fragrance Association (CTFA) method, and the linear regression method. Although the procedures used in these methods are similar, the times at which samples are analyzed and the interpretation of test results--the acceptance criteria by which products are judged to be effectively preserved--are different. Selection of the method used depends on a number of considerations, including the time available for testing, the reliability of the method, the ability to maintain control of the method by use of statistical treatment of the data, the "need" for rechallenge testing, the type of product to be examined, the test organisms used, and acceptance criteria.
It would be desirable to have a standardized means of interpreting data from all types of preservative efficacy tests. This would enable one to compare the results obtained using different test methods. In this article, the acceptance criteria of the USP and CTFA methods were converted to decimal reduction times (D-values) so that these rates of killing test organisms could be compared with each other and to the acceptance criteria of the linear regression method, which uses D-values. Then, the D-values that constituted the acceptance criteria of these methods were used to determine the adequacy of preservation of emulsions that were inadequately preserved, marginally preserved, and satisfactorily preserved.
It was found that the acceptance criteria of the linear regression method were required to ensure that products were satisfactorily preserved. Products that meet the acceptance criteria of the USP and CTFA methods (and which do not meet the acceptance criteria of the linear regression method) occasionally fail to prevent the growth of microorganisms.
The Acceptance criteria of the USP, CTFA and linear regression methods are outlined here.
USP Method: The preservative is effective in the product examined if:
1. The concentrations of viable bacteria are reduced
to not more than 0.1% of the initial concentrations
by the fourteenth day.
2. The concentrations of viable yeasts and molds
remain at or below the initial concentrations during
the first 14 days.
3. The concentration of each test microorganism
remains at or below these designated levels during
the remainder of the 28-day test period.
CTFA Method: The CTFA guideline for preservation testing of aqueous liquid and semiliquid eye cosmetics recommends the following criteria for preservative efficacy assessment:
1. There should be greater than 99.9% reduction of
vegetative bacteria within …