3 research outputs found
Robust Fit of Toxicokinetic–Toxicodynamic Models Using Prior Knowledge Contained in the Design of Survival Toxicity Tests
Toxicokinetics–toxicodynamic
(TKTD) models have emerged
as a powerful means to describe survival as a function of time and
concentration in ecotoxicology. They are especially powerful to extrapolate
survival observed under constant exposure conditions to survival predicted
under realistic fluctuating exposure conditions. But despite their
obvious benefits, these models have not yet been adopted as a standard
to analyze data of survival toxicity tests. Instead simple dose–response
models are still often used although they only exploit data observed
at the end of the experiment. We believe a reason precluding a wider
adoption of TKTD models is that available software still requires
strong expertise in model fitting. In this work, we propose a fully
automated fitting procedure that extracts prior knowledge on parameters
of the model from the design of the toxicity test (tested concentrations
and observation times). We evaluated our procedure on three experimental
and 300 simulated data sets and showed that it provides robust fits
of the model, both in the frequentist and the Bayesian framework,
with a better robustness of the Bayesian approach for the sparsest
data sets
Robust Fit of Toxicokinetic–Toxicodynamic Models Using Prior Knowledge Contained in the Design of Survival Toxicity Tests
Toxicokinetics–toxicodynamic
(TKTD) models have emerged
as a powerful means to describe survival as a function of time and
concentration in ecotoxicology. They are especially powerful to extrapolate
survival observed under constant exposure conditions to survival predicted
under realistic fluctuating exposure conditions. But despite their
obvious benefits, these models have not yet been adopted as a standard
to analyze data of survival toxicity tests. Instead simple dose–response
models are still often used although they only exploit data observed
at the end of the experiment. We believe a reason precluding a wider
adoption of TKTD models is that available software still requires
strong expertise in model fitting. In this work, we propose a fully
automated fitting procedure that extracts prior knowledge on parameters
of the model from the design of the toxicity test (tested concentrations
and observation times). We evaluated our procedure on three experimental
and 300 simulated data sets and showed that it provides robust fits
of the model, both in the frequentist and the Bayesian framework,
with a better robustness of the Bayesian approach for the sparsest
data sets
Statistical Handling of Reproduction Data for Exposure-Response Modeling
Reproduction
data collected through standard bioassays are classically
analyzed by regression in order to fit exposure-response curves and
estimate EC<sub><i>x</i></sub> values (<i>x</i>% effective concentration). But regression is often misused on such
data, ignoring statistical issues related to (i) the special nature
of reproduction data (count data), (ii) a potential inter-replicate
variability, and (iii) a possible concomitant mortality. This paper
offers new insights in dealing with those issues. Concerning mortality,
particular attention was paid not to waste any valuable dataî—¸by
dropping all the replicates with mortalityî—¸or to bias EC<sub><i>x</i></sub> values. For that purpose we defined a new
covariate summing the observation periods during which each individual
contributes to the reproduction process. This covariate was then used
to quantify reproductionî—¸for each replicate at each concentrationî—¸as
a number of offspring per individual-day. We formulated three exposure-response
models differing by their stochastic part. Those models were fitted
to four data sets and compared using a Bayesian framework. The individual-day
unit proved to be a suitable approach to use all the available data
and prevent bias in the estimation of EC<sub><i>x</i></sub> values. Furthermore, a nonclassical negative-binomial model was
shown to correctly describe the inter-replicate variability observed
in the studied data sets