Recommended criteria for the evaluation of bacterial mutagenicity data (Ames test)

• Published in: Mutation research Vol. 848; p. 403074
• Main Authors: Levy, Dan D., Zeiger, Errol, Escobar, Patricia A., Hakura, Atsushi, van der Leede, Bas-jan M., Kato, Masayuki, Moore, Martha M., Sugiyama, Kei-ichi
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2019
• Subjects: Ames; Animals; Evaluation Studies as Topic; Humans; Interpretation criteria; Mutagenicity Tests; Regulatory testing; Salmonella typhimurium - genetics; Ames; Regulatory testing; Interpretation criteria
• ISSN: 1383-5718; 1879-3592; 1873-135X
• DOI: 10.1016/j.mrgentox.2019.07.004

•Identification of factors to be considered when evaluating Ames test data.•The relative strengths and weaknesses of currently used evaluation criteria.•The use of a combination of criteria when evaluating test data.•The need for repeat testing using the same or modified experimental conditions.•The implementation and use of solvent and positive control charts. A committee was constituted within the International Workshop on Genetic Toxicology Testing (IWGT) to evaluate the current criteria for a valid Ames test and to provide recommendations for interpretation of test results. Currently, determination of a positive vs. a negative result is made by applying various data evaluation procedures for comparing dosed plates with the concurrent solvent control plates. These evaluation procedures include a requirement for a specific fold increase (2- or 3-fold, specific to the bacterial strain), formal statistical procedures, or subjective (expert judgment) evaluation. After extensive discussion, the workgroup was not able to reach consensus recommendations in favor of any of these procedures. There was a consensus that combining additional evaluation criteria to the comparison between dosed plates and the concurrent solvent control plates improves test interpretation. The workgroup recommended using these additional criteria because the induction of mutations is a continuum of responses and there is no biological relevance to a strict dividing line between a positive (mutagenic) and not-positive (nonmutagenic) response. The most useful additional criteria identified were a concentration-response relationship and consideration of a possible increase above the concurrent control in the context of the laboratory’s historical solvent control values for the particular tester strain. The workgroup also emphasized the need for additional testing to resolve weak or inconclusive responses, usually with altered experimental conditions chosen based on the initial results. Use of these multiple criteria allowed the workgroup to reach consensus on definitions of “clear positive” and “clear negative” responses which would not require a repeat test for clarification. The workgroup also reached consensus on recommendations to compare the responses of concurrent positive and negative controls to historical control distributions for assay acceptability, and the use of control charts to determine the validity of the individual test.