Challenges and approaches to conducting and interpreting the amphibian metamorphosis assay and the fish short-term reproduction assay.

The amphibian metamorphosis assay (AMA) and the fish short-term reproduction assay (FSTRA) are screening assays designed to detect potential endocrine activity of a test substance. These assays are included in a battery of assays in Tier 1 of U.S. Environmental Protection Agency's Endocrine Disruptor Screening Program. Based on our laboratory's experience with these two assays, we have noted several challenges in the conduct and interpretation of the AMA and FSTRA, including, but not limited to, diseased/parasitized test organisms, failure to meet some guideline performance criteria, and issues selecting and maintaining test concentrations. Various approaches are described for addressing the challenges associated with both the conduct and interpretation of these assays. Historical control data for both the AMA and FSTRA are presented to further understand background occurrences of histopathological phenomena and variability associated with the measured endpoints in these assays. In the historical control database for the AMA, wet weight on day 7 was the most variable endpoint (coefficient of variation = 26%), while developmental stage on day 21 was least variable (coefficient of variation = 0.47%). In the FSTRA, vitellogenin concentrations were the most variable endpoint (coefficient of variation = 47-84%), while fertility was the least variable endpoint (coefficient of variation = 1.5%) among historical controls.