ABILITY OF LABORATORY METHODS TO PREDICT IN-USE EFFICACY OF ANTIMICROBIAL PRESERVATIVES IN AN EXPERIMENTAL COSMETIC

The abilities of nine antimicrobial systems to preserve an experimenta l water-based cosmetic formulation were evaluated by six microbiologic al challenge tests: the U. S. Pharmacopeia test; the British Pharmacop eia test; the Cosmetic, Toiletry, and Fragrance Association test; the rapid screen test; the sequential challenge test; and the post-use tes t. The antimicrobial systems contained various combinations and amount s of two parabens and a quaternary compound in order to provide a broa d range of preservation. The results obtained were compared with the a bilities of the formulations to support maintenance and growth of micr oorganisms in microfloras obtained from human axilla areas and finger skin during an 8-week simulated in-use test. Without statistical analy sis all of the tests predicted the results obtained with well-preserve d or poorly preserved formulations. The rapid screen test was the best test for predicting differences at intermediate levels of preservatio n. Statistically, all of the tests were equivalent predictors of prese rvation efficacy in the in-use test (P = 0.05). At the P = 0.10 level, only the U.S. Pharmaceopeia, British Pharmacopeia, rapid screen, Cosm etic, Toiletry, and Fragrance Association tests were significantly pre dictive. The results of prediction by a test, based on the preservativ e levels used, agreed well with the in-use test results (P = 0.01). A total of 20% of the formulations that contained excessive microbial le vels contained human axilla microorganisms. The levels of preservation in failed products were similar to the levels of preservation in unus ed controls.