Ambiguous results in genotoxicity: beyond “positive / negative”

An Ames test or a micronucleus test does not always deliver a clear-cut verdict. When faced with an ambiguous result, the right question is not “positive or negative?”, but “is this signal biologically real?”

A signal that is hard to read, not a verdict

An ambiguous (equivocal) result arises when the data allow us neither to conclude that there is genotoxic activity, nor to rule it out. The causes vary: a small increase, a response at a single concentration, variability close to the laboratory’s usual values, marked cytotoxicity, solubility difficulties.

A graduated approach, test by test

We first analyse the existing data before considering any new experiment.

• Ames test (OECD 471): magnitude of the increase, position relative to historical controls, dose-response relationship, cytotoxicity, precipitation, effect of metabolic activation. If needed, a targeted experiment clarifies the signal.
• In vitro micronucleus test (OECD 487): validity of the controls, historical values, level of cytotoxicity, reproducibility.
• FISH analysis with centromeric probes: it shows whether micronuclei contain a whole chromosome (aneugenic lead) or a fragment (clastogenic lead), and so sheds light on the mechanism.

This analysis enriches the interpretation. It does not replace it.

Three outcomes, one reasoned conclusion

Depending on the investigations carried out, three situations arise:

In every case, the conclusion rests on the weight of evidence: Ames, micronucleus, chemical structure, structural alerts and literature data are read together.

Read the white paper

Our role is not limited to the technical performance of studies: it includes interpretation and support for the decisions that follow. The white paper details our method, examples for the Ames and micronucleus tests, the contribution of FISH analysis and our process for engaging with your toxicologist.

Download the white paper — available in English and French.