COSMETIC PRESERVATION 203 A major difference between PETs is due to a lack of understanding of the purpose of the PET. Defining the purpose of the test is critical. The entire experimental design for validating the PET will differ depending on the definition of purpose. The experimental design for validating use of a PET as a predictor of the potential for consumer contam- ination is different from that for validating use of a PET to demonstrate the presence of the preservative or as a predictor of potential for manufacturing contamination. To validate a PET as a predictor of consumer contamination requires prospective correlative consumer studies or retrospective validation, based on lack of consumer complaints, to corroborate the PET laboratory results. Regardless of which philosophy one adopts to define the purpose of a PET, at a minimum the goal should be to develop a data base to rank the antimicrobial hostility of the company's products. The purpose of a PET as viewed by the FDA is to predict consumer contamination potential (59,60). The FDA has tried several times to develop a PET for this purpose without success. Products failing such a test would be subject to recall. Despite the collaborative work between CTFA, AOAC, and FDA to develop a standardized PET complete with multi-lab comparisons and statistical analysis, the method has not yet been demonstrated to have the ability to predict a product's ability to withstand mi- crobial insult that may occur during intended use, since no correlative consumer studies were conducted using the same products for which the PET was conducted. Such an omission is fortunate. If such a standard PET were developed that was predictive of consumer contamination, then it could be used to enforce a recall on those products that fail it. One could counter the recall by pointing out that a PET does not account for consumer use and packaging parameters. One might also counter this argument with the observation that if the PET is done on freshly made product, then the PET data would only apply to freshly made product. Since most companies conduct PETs on shelf-aged product, such a statement would be admitting that they are out of line with the majority of reputable companies and have products that become a risk over time. Several publications have already shown that a modified version of the CTFA preserva- tive efficacy test is a valid predictive model of the risk of consumer contamination (17-20), but these all used proprietary "in-house" organisms unavailable to others. Thus, the CTFA test described by these publications does not provide a standard PET that could be used to enforce a recall as described above. Another study has compared several PETs for the ability to predict "in-use" contamination (61). The major criticism of this work is that the in-use test was merely simulated. The subjects dabbled with the product after rubbing their underarms with their fingers. The significance of ranking PETs against their ability to predict how well a product can withstand simulated consumer use does not represent validation against true in-use conditions. Nevertheless, since all the PETs were ranked against a single standard, one can still derive considerably useful information. For example, nearly all the compendial tests adequately separated poorly preserved from well-preserved products. Some of the more conservative tests classified marginally preserved products the same as poorly preserved ones, while the more liberal tests allowed marginally preserved products to rank with well preserved ones. The CTFA test exhibited the tendency to rank all three (poor, marginal, and well) correctly against the flawed but useful standard of a simulated in-use test. This study does not, however, support the use of the CTFA test to enforce recalls, since the comparison was against an invalid simulated in-use test.

204 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS COMMON ISSUES SHARED BY PETS PETs are conceptually simple. All the protocols involve introducing microorganisms into the cosmetic. All the protocols have the same general steps: product preparation, inoculum selection and preparation, inoculation, incubation, plating and estimating the surviving microorganisms, and interpreting the data. The differences are in how these steps are conducted. These differences are described below. Product preparation. The product sample used in a PET should replicate all the param- eters for national distribution, including formulation, packaging, and manufacturing conditions. Raw materials should be of the same quality and from the same source as expected for national distribution. Even minor changes for adjusting viscosity, color, perfume, pH--even changes in process water--may adversely affect a product's PET results. Scale-up from lab to plant also provides opportunity for variables that need to be identified and controlled so the final nationally delivered product will be adequately preserved. Most published preservative methods test full-strength (100%) product (15). However, Brannan et al. (17, 18) and Cooke et al. (62) added diluted product to the challenge test as well. The dilutions stress the product and also provide a means of ranking the product. For example, products that can be diluted and continue killing the inoculum may be classified as well-preserved, whereas products that kill the inoculum only when the product is at full strength may be adequately preserved if the packaging prevents consumer contamination during use. In addition, this approach is another way of mimicking expected use patterns. For example, products that are diluted during fore- seeable misuse, such as shampoos, should be able to continue killing microbial chal- lenges. Finally, dilution mimicks potential manufacturing errors, particularly those involving washouts where diluted product is accidently left in a line. If a product can remain hostile when diluted, then microorganisms are less likely to be selected to be resistant in the biocide. If not, then the organisms are selected for survival at diluted biocide concentrations and are just a minor step away from being selected for growth in full-strength product. Inoculum: Selection and maintaining resistance. An appropriate microbiological challenge of the product is the most critical factor in determining the validity of a preservative efficacy test. All the tests currently specify a set of inoculum microorganisms. Some of the methods list specific strains from ATCC, while others also allow inclusion of other organisms the microbiologist chooses. These choices often include preservative-resistant strains from consumer-used product samples, raw materials, or manufacturing sites. However, use of these resistant organisms may be considered a form of abuse testing by some. Use of these special strains, however, should be reevaluated if one has not maintained a rigorous program of preserving the originally isolated culture. More often than not, one maintains a culture collection by putting up an original set of vials. When the last remaining vial is subcultured, an isolated colony (obtained by streaking for isolation) is selected to grow up and harvest. This culture is preserved in another set of lyophilized vials. Unfortunately, this process represents a departure from the originally deposited culture because the progeny of only a single individual was selected to represent the original population. Routine subculturing on nonselective growth media will also cause the loss of preser- vative resistance since the selective pressure of the preservative is no longer present. The