Alternatives to statistical decision trees in regulatory (eco-)toxicological bioassays

The goal of (eco-) toxicological testing is to experimentally establish a dose or concentration-response and to identify a threshold with a biologically relevant and probably non-random deviation from "normal". Statistical tests aid this process. Most statistical tests have distributional assumptions that need to be satisfied for reliable performance. Therefore, most statistical analyses used in (eco-)toxicological bioassays use subsequent pre- or assumption-tests to identify the most appropriate main test, so-called statistical decision trees. There are however several deficiencies with the approach, based on study design, type of tests used and subsequent statistical testing in general. When multiple comparisons are used to identify a non-random change against negative control, we propose to use robust testing, which can be generically applied without the need of decision trees. Visualization techniques and reference ranges also offer advantages over the current pre-testing approaches. We aim to promulgate the concepts in the (eco-) toxicological community and initiate a discussion for regulatory acceptance.

Analysis of historical negative control group data from the rat in vivo micronucleus assay

A database of micronuclei counts for historical negative control data from rat in vivo micronuclei tests performed in 10 different laboratories was established. Data were available from over 4000 negative control rats from 10 laboratories. The mean frequency of micronucleated cells (MN)/1000 cells ranged from 0.44 to 2.22, a 5-fold range. Overall there were no major sex or strain differences in frequency, although there were some small but statistically significant differences within laboratories. There was appreciable variability between experiments compared with variability within experiments in some laboratories. No specific factor was identified which could explain this variability although it was noted that many different vehicles were used in the experiments. It is hoped that these data will help laboratories beginning studies with the rat micronucleus assay and those involved in the assessment of micronucleus assay results.

Use of Lung Weight as Biomarker for Assessment of Lung Toxicity in Rat Inhalation Studies

Subacute inhalation study (1 week or 2 weeks) is an important process for screening out inhaled compounds causing lung irritation. To investigate whether the lung weight can be used as an indicator for acute lung injury, we have analyzed retrospectively the lung weight data from 30 studies in rats exposed to dry powder inhalation. The lung weight change was correlated with lung histopathology in the majority of studies (25 of 30), showing as either both changed or both unchanged. The sensitivity and specificity of using the weight change in lungs as biomarker for predicting lung histopathology in these studies were over 80%. The pattern of lung weight change was often similar in the 1- to 2- week studies and the 4-week studies. Our analysis of covariance model showed that a study with 40 rats (5 males + 5 females/group and 4 groups) could detect lung weight change greater than 10% to 20% of control value. These results suggested that lung weight is a useful indicator for identifying acute lung toxic effect by inhaled compounds in these subacute inhalation studies.