Statistical results and implications of the enchytraeid reproduction ringtest.

An international ringtest of the enchytraeid reproduction test (ERT) was performed by 29 laboratories, with two substances (carbendazim and 4-nitrophenol) and two test designs (NOEC and ECx). Although many participants had no previous experience in enchytraeid ecotoxicology, the majority of tests were performed accurately and fulfilled the validity criteria, demonstrating method feasibility. However, variability in control reproduction was fairly high and exceeded the proposed validity criterion of a 50% coefficient of variation in 24% of the tests. Variability may be lowered with increasing experience and by using better defined animals, either through synchronized culture or by weighing animals before the test. In all tests, NOECs were higher than the EC10 values, sometimes by more than 1 order of magnitude and usually closer to the EC50. This shows that NOECs, due to their dependence on test design and variability, are an unsatisfactory measure of "no toxicity" for regulatory purposes. Preferably, differences to the control at NOEC concentrations should always be given. To achieve appropriate power and to avoid frequent false negatives, in the ERT, high numbers of replicates would be necessary. We favor using regression approaches with means of replicates whenever possible.