title: Statistically significant deviations from additivity: What do they mean in assessing toxicity of mixtures?
abstract: There is increasing attention from scientists and policy makers to the joint effects of multiple metals on organisms when present in a mixture. Using root elongation of lettuce (Lactuca sativa L.) as a toxicity endpoint, the combined effects of binary mixtures of Cu, Cd, and Ni were studied. The statistical MixTox model was used to search deviations from the reference models i.e. concentration addition (CA) and independent action (IA). The deviations were subsequently interpreted as 'interactions'. A comprehensive experiment was designed to test the reproducibility of the 'interactions'. The results showed that the toxicity of binary metal mixtures was equally well predicted by both reference models. We found statistically significant 'interactions' in four of the five total datasets. However, the patterns of 'interactions' were found to be inconsistent or even contradictory across the different independent experiments. It is recommended that a statistically significant 'interaction', must be treated with care and is not necessarily biologically relevant. Searching a statistically significant interaction can be the starting point for further measurements and modeling to advance the understanding of underlying mechanisms and non-additive interactions occurring inside the organisms.
authors.affiliation: Institute of Environmental Sciences (CML), Leiden University, 2300 RA Leiden, The Netherlands. Electronic address: liu@cml.leidenuniv.nl.
authors.forename: Yang
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Institute of Environmental Sciences (CML), Leiden University, 2300 RA Leiden, The Netherlands.
authors.forename: Martina G
authors.initials: MG
authors.lastname: Vijver
authors.affiliation: Institute of Environmental Sciences (CML), Leiden University, 2300 RA Leiden, The Netherlands.
authors.forename: Hao
authors.initials: H
authors.lastname: Qiu
authors.affiliation: Centre for Ecology & Hydrology (CEH), MacLean Building, Benson Lane, OX10 8BB Wallingford, Oxfordshire, United Kingdom.
authors.forename: Jan
authors.initials: J
authors.lastname: Baas
authors.affiliation: Institute of Environmental Sciences (CML), Leiden University, 2300 RA Leiden, The Netherlands; National Institute of Public Health and the Environment (RIVM), Center for Safety of Substances and Products, 3720 BA Bilthoven, The Netherlands.
authors.forename: Willie J G M
authors.initials: WJ
authors.lastname: Peijnenburg
mesh_terms: D002104:Cadmium / Q000633:toxicity*; D003300:Copper / Q000633:toxicity*; D004357:Drug Synergism; D018545:Lactuca / Q000187:drug effects* / Q000254:growth & development; D008962:Models, Theoretical; D009532:Nickel / Q000633:toxicity*; D018517:Plant Roots / Q000187:drug effects / Q000254:growth & development; D015203:Reproducibility of Results
chemical_list: D002104:Cadmium; D003300:Copper; D009532:Nickel
keywords: Biologically relevant; Lettuce; Metal mixtures; Reproducibility; Statistically significant