title: The Ames test: the two-fold rule revisited.
abstract: Mutagenicity in the Ames assay is evaluated by comparing the number of revertants observed in treated cultures to those in untreated cultures. Often, some form of the '2-fold rule' is employed, whereby a compound is judged mutagenic if a 2-fold or greater increase is seen in a treated culture. In order to understand the underpinnings of this approach, we study some of its statistical properties. We assume that the number of revertants on any plate from a given two-group experiment follows a Poisson distribution and we address the following questions: (1) what is the false-positive error probability of observing at least a doubling of the number of colonies from the control to the treatment group?; (2) if a given mean number of colonies is postulated for a control group, what number of colonies above the observed control mean provides a false-positive rate of 5%? We also present results for question 1 in the case where the number of revertants follows a negative binomial distribution.
authors.affiliation: Glaxo Wellcome Inc., Medicines Safety Evaluation Division, Research Triangle Park, NC 27709, USA.
authors.forename: N F
authors.initials: NF
authors.lastname: Cariello
authors.forename: W W
authors.initials: WW
authors.lastname: Piegorsch
mesh_terms: D005189:False Positive Reactions; D009152:Mutagenicity Tests / Q000592:standards*; D016012:Poisson Distribution; D012486:Salmonella typhimurium / Q000235:genetics