Slide preparation and sampling as a major source of variability in the mouse micronucleus assay

This paper describes the results of a study in which mouse bone marrow micronucleus assay slides were assessed for homogeneity of micronucleated polychromatic erythrocytes (MPE) among polychromatic erythrocytes (PE). The slides were prepared by 3 distinct methods and several methods of slide reading were assessed. Observations made using our slides were confirmed by re-analysis of slides from 3 independent laboratories. It is concluded that the method of slide preparation and assessment can significantly influence the variability of data obtained from a study. The extent of this variability casts doubt upon the validity of certain assumptions concerning this assay--such as sex differences in MPE incidence, responder variability, etc. Results are discussed within the context of the very recent literature for this assay. Some laboratories appear to have adequate methods of slide preparation and data accumulation, while others do not. Methods to improve the sensitivity of this assay are suggested within the context of the recommendations made by the Gene-Tox review group. In particular, it is suggested that individual investigators present evidence of the adequacy of their data accumulation techniques in order to enhance the value of future studies.