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The Ames test: the two-fold rule revisited

N F Cariello¹, W W Piegorsch

¹Glaxo Wellcome Inc., Medicines Safety Evaluation Division, Research Triangle Park, NC 27709, USA.

Mutation Research

|July 10, 1996

Summary

This study statistically analyzes the Ames assay's "2-fold rule" for mutagenicity. It quantifies false-positive rates under Poisson and negative binomial distributions, crucial for accurate genotoxicity testing.

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Area of Science:

Toxicology
Genetics
Biostatistics

Background:

Mutagenicity testing relies on comparing treated vs. untreated cultures in the Ames assay.
The '2-fold rule' is a common heuristic for identifying mutagenic compounds.
Statistical properties of this rule require rigorous examination for reliable genotoxicity assessment.

Purpose of the Study:

To statistically evaluate the '2-fold rule' used in Ames assay mutagenicity testing.
To determine the false-positive error probability associated with a 2-fold increase in revertants.
To establish a threshold for a 5% false-positive rate based on control group means.

Main Methods:

Statistical modeling of revertant counts using Poisson distribution.
Statistical modeling of revertant counts using negative binomial distribution.
Calculation of false-positive error probabilities and determination of critical thresholds.

Main Results:

The study quantifies the false-positive error rate for the '2-fold rule' under different statistical distributions.
Results provide insights into the probability of falsely identifying a compound as mutagenic.
A method is presented to determine the number of revertants indicative of mutagenicity at a 5% false-positive rate.

Conclusions:

The statistical properties of the '2-fold rule' are essential for accurate interpretation of Ames assay results.
Understanding these properties helps in optimizing genotoxicity testing protocols.
This work provides a statistical framework for more reliable mutagenicity assessments.

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