FETAX INTERLABORATORY VALIDATION-STUDY - PHASE-II TESTING

The Frog Embryo Teratogenesis Assay-Xenopus (FETAX) is a 96-h whole em bryo developmental toxicity screening assay that can be used in ecotox icology and in detecting mammalian developmental toxicants when an in vitro metabolic activation system is employed. A standardized American Society for Testing and Materials (ASTM) guide for the conduct of FET AX has been published along with a companion atlas that helps in embry o staging and identifying malformations. As part of the ASTM process, an interlaboratory validation study was undertaken to evaluate the rep eatability and reliability of FETAX. Six different laboratories partic ipated in the study. Each laboratory utilized one technician with the exception of one laboratory, which utilized two independent technician s. In Phase I, FETAX proved to be more repeatable and reliable than ma ny other bioassays. However, some excessive variation was observed in a few laboratories. Some of this variation may have been due to an ini tial lack of experience with the assay by some technicians. Phase II, which is reported here, showed far less intralaboratory and interlabor atory variability than did Phase I. Nonteratogens such as saccharin an d sodium cyclamate showed the most consistent results, whereas more va riability was observed for the teratogens caffeine and 5-fluorouracil. Interlaboratory coefficient of variation values for all FETAX end poi nts ranged from 7.3 to 54.7%. The minimum concentration to inhibit gro wth proved to be the most variable end point for three of the four tes t chemicals, whereas the LC50 and EC50 (malformation) proved to be les s variable.