Evaluating Laboratory Performance in a Proficiency Test for Antimicrobial Protection of a Cosmetic Formulation

Accurate assessments of antimicrobial protection of cosmetic products play a crucial role in monitoring their quality and user health. The reliability of these analyses depends on the method used and the competence of cosmetic laboratories. This paper presents the design and results of a proficiency test organized in November 2023 to assess the performance of 37 laboratories participating in the determination of the logarithmic reduction in Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans, and Aspergillus brasiliensis 7 days after inoculation in a cosmetic formulation. Participating laboratories performed their duplicate analysis on each test microorganism under repeatability conditions, using the same analytical method. In order to evaluate the consistency of laboratory performance, two trials were scheduled at a one-week interval, and they were conducted by the same analyst, applying the same analytical procedure on identical samples. The results of this study demonstrated the overall good application of the method in the determination of microbial reduction by participating laboratories and highlighted the robustness of integrating method precision assessment into proficiency testing for monitoring the analytical performance of cosmetic microbiology laboratories.