International guidelines for cytotoxicity limits for the in vitro chromosomal aberration assay require reductions in cell growth of greater than 50%. This sets no upper limit on toxicity and there is concern about the number of false or irrelevant results obtained in the aberration assay, i.e., positive results at toxic dose levels only, with no evidence for primary DNA damaging ability and with negative results in the other genotoxicity tests. We have previously proposed that no truly genotoxic compound would be missed if the toxicity of the highest dose did not exceed 50%. Cell growth measured by cell counts as a percentage of controls can underestimate toxicity. For example, if we seed half a million cells per culture, and the controls double to 1 million during the experiment, a culture that truly has no growth will still have a cell count 50% of the control. Measurement of population doublings (PDs) more accurately assesses cell growth. To assess the use of PD in dose selection, we examined previous data from this lab and data from new experiments with "true," primary DNA damaging clastogens, and with clastogens, including drugs, thought to act indirectly, through cytotoxicity-associated mechanisms. We compared aberration results where the highest doses scored were based on 50% reductions in final cell counts with results obtained when the highest doses were based on PD. The PD method allows detection of true clastogens, including those that are active in a range with some toxicity, and reduces the number of toxicity-related "false"-positive results.

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