ORIGINAL RESEARCH Determination of mutagenicity of the precipitate formed by sodium hypochlorite and chlorhexidine using the Ames test Pranali Patil, BDS, MSD 1 ; Anita Aminoshariae, DDS, MS 1 ; Jarrod Harding, PhD 2 ; Thomas A Montagnese, DDS, MS 1 ; and Andre Mickel, DDS, MSD 1 1 Department of Endodontics, Case School of Dental Medicine, Cleveland, Ohio, USA 2 Department of Biostatistics, Case Western Reserve University, Cleveland, Ohio, USA Keywords Carcinogenicity, chlorhexidine, endodontic, irrigants, mutagenicity, root canal. Correspondence Dr Anita Aminoshariae, Diplomate of American Board of Endodontics, 2124 Cornell Rd, Cleveland, OH 44106, USA. Email: axa53@case.edu doi:10.1111/aej.12100 Abstract The aim of this study was to determine the direct mutagenic potential of any precipitate formed by combining sodium hypochlorite (NaOCl) and chlorhexidine (CHX). The precipitates formed by NaOCl and CHX were dis- solved in 100% dimethyl sulfoxide and cultured with mutant Salmonella Typhimurium strains. The cells were observed for reverse mutation. The numbers of positive/mutated wells were statistically compared with those in the background plates using the two-sample proportion independent t-test. The precipitates were not found to be significantly more mutagenic than the background plates. Within the limitations of this study, the results suggest that the precipitates formed when sodium hypochlorite and chlorhexidine contact did not show mutagenic (and are therefore carcinogenic) potential. Introduction A mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level (1). As many mutations can result in cancer, mutagens are therefore likely to be car- cinogens (2). Many test systems have been devised to screen for mutagenicity and/or carcinogenicity (3). As carcinogenicity tests are expensive and time consuming (4), rapid tests were developed that made use of microbes, such as Salmonella Typhimurium and Escherichia coli, and tested for mutagenicity rather than carcinogenic- ity (5,6). The authors reported that the mutants of S. Typhimurium could detect and classify chemical muta- gens with great simplicity and sensitivity for the initial phase of mutagenicity testing. As DNA is chemically the same in all organisms, any living organism can be used to test for mutagens (7). An understanding of the specificity of mutagens in bacteria has led to their direct implication as causative factors of human cancers (8). McCann et al. reported that 90% of the carcinogens tested were mutagens and 90% of the non-carcinogens were non-mutagens (9). In his study, Zeiger showed that a clear mutagenic or equivocal muta- genic response in Salmonella was predictive for 77% of the carcinogens or equivocal carcinogens in laboratory animals and that there is a high predictive value for rodent carcinogenicity when a mutagenic response is obtained in bacterial tests (10). After decades of testing and investiga- tions (2,5,11–13), the authors reported that the chemicals to which humans are exposed, which are clearly positive in the bacterial test, should be considered potential human health hazards. An example of the sensitivity of the Ames test is the study on mutagenicity of cigarette smoke, wherein the authors found that smoke condensate from less than 0.01 of one cigarette could be detected (14). The test has also been successfully used worldwide to test for genotoxic materials in water bodies (13,15) and is claimed to be one of the standard methods for water and wastewater (16). Watanabe et al. confirmed that S. Typhimurium strains TA100 and TA98 were extremely © 2015 Australian Society of Endodontology 16 Aust Endod J 2016; 42: 16–21