BIOSTATISTICAL ANALYSIS OF THE MICRONUCLEUS MUTAGENICITY ASSAY BASED ON THE ASSUMPTION OF A MIXING DISTRIBUTION

The in vivo micronucleus assay can be analyzed by comparing the number of micronuclei (MN) of several dose groups with those of a control gr oup. In several publications, difficulties arose in estimating a suita ble distribution for MN, even in the untreated historical control grou ps. Mitchell et al. described the presence of a subpopulation of more susceptible responders. Based on this assumption of such a subpopulati on, score tests were used for the mixing distribution of responders an d nonresponders (behavior same as in untreated control animals) within the dose groups. The power behavior of these tests was characterized with a simulation study. The advantage of score tests can be shown, ev en in the practical and important guideline case of only five animals per group.