The Ames test: the two-fold rule revisited

Mutat Res. 1996 Jul 10;369(1-2):23-31. doi: 10.1016/s0165-1218(96)90044-0.


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.

MeSH terms

  • False Positive Reactions
  • Mutagenicity Tests / standards*
  • Poisson Distribution
  • Salmonella typhimurium / genetics