This report summarizes the proceedings of the September 9–10, 2005 meeting of the Expert Working Group on Hazard Identification
and Risk Assessment in Relation to In Vitro Testing, part of an initiative on genetic toxicology. The objective of theWorking
Group was to develop recommendations for interpretation of results from tests commonly included in regulatory genetic toxicology
test batteries, and to propose an appropriate strategy for follow-up testing when positive in vitro results were obtained in these
assays. The Group noted the high frequency of positive in vitro findings in the genotoxicity test batteries with agents found not to be
carcinogenic and thought not to pose a carcinogenic health hazard to humans. The Group agreed that a set of consensus principles for
appropriate interpretation and follow-up testing when initial in vitro tests are positive was needed. Current differences in emphasis
and policy among different regulatory agencies were recognized as a basis of this need. Using a consensus process among a balanced
group of recognized international authorities from industry, government, and academia, it was agreed that a strategy based on these
principles should include guidance on: (1) interpretation of initial results in the “core” test battery; (2) criteria for determining when
follow-up testing is needed; (3) criteria for selecting appropriate follow-up tests; (4) definition of when the evidence is sufficient to
define the mode of action and the relevance to human exposure; and (5) definition of approaches to evaluate the degree of health
risk under conditions of exposure of the species of concern (generally the human).
A framework for addressing these issues was discussed, and a general “decision tree” was developed that included criteria for
assessing the need for further testing, selecting appropriate follow-up tests, and determining a sufficient weight of evidence to
attribute a level of risk and stop testing. The discussion included case studies based on actual test results that illustrated common