title: Defining the baseline for inhibition concentration calculations for hormetic hazards.
abstract: The use of endpoint estimates based on modeling inhibition of test organism response relative to a baseline response is an important tool in the testing and evaluation of aquatic hazards. In the presence of a hormetic hazard, the definition of the baseline response is not clear because non-zero levels of the hazard stimulate an enhanced response prior to inhibition. In the present study, the methodology and implications of how one defines a baseline response for inhibition concentration estimation in aquatic toxicity tests was evaluated. Three possible baselines were considered: the control response level; the pooling of responses, including controls and all concentration conditions with responses enhanced relative to controls; and, finally, the maximal response. The statistical methods associated with estimating inhibition relative to the first two baseline definitions were described and a method for estimating inhibition relative to the third baseline definition was derived. These methods were illustrated with data from a standard aquatic zooplankton reproductive toxicity test in which the number of young produced in three broods of a cladoceran exposed to effluent was modeled as a function of effluent concentration.
authors.affiliation: Department of Maths and Statistics, Center for Environmental Toxicology and Statistics, Miami University, OH 45045, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Bailer
authors.forename: J T
authors.initials: JT
authors.lastname: Oris
mesh_terms: D000222:Adaptation, Physiological / Q000187:drug effects*; D000818:Animals; D003621:Daphnia; D004305:Dose-Response Relationship, Drug; D005260:Female; D008954:Models, Biological; D012098:Reproduction / Q000187:drug effects; D018675:Toxicity Tests; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D014874:Water Pollutants, Chemical