Methods and criteria for deciding whether specific-locus mutation-rate data in mice indicate a positive,negative, or inconclusive result

The binomial approximation of the UMPU (uniformly most powerful unbiased) test for the equality of 2 binomial proportions is shown to be a highly accurate and easily applied method for testing the hypothesis that a given mouse specific-locus mutation frequency is not higher than the spontaneous mutation frequency (43 mutations in 801 406 offspring, for males). Critical sample sizes have been calculated that show at a glance whether P < 0.05.The first hypothesis that the mutation frequency (induced + spontaneous) of treated mice is not higher than the spontaneous mutation frequency is combined with the second hypothesis that the induced mutation frequency of treated mice is no less than 4 times the historical-control mutation frequency to produce a multiple decision procedure with 4 possible decisions: inconclusive result, negative result, positive result, and weak mutagen. Critical sample sizes for the second hypothesis, also with P < 0.05, are combined with those for the first hypothesis into a grid that permits rapid evaluation of data according to these criterea. The justification for using these criteria in reaching decisions, assuming a high level of exposure has been given, is the practical necessity of rapidly determining which chemicals are potent mutagens.Positive results can become apparent in relatively small samples. Larger samples, of at least 11 166 offspring, are required to obtain a negative result. If samples of 18 000 are routinely collected (unless positive results are found earlier), 75% of tests of chemicals that are non-mutagens will give a negative result. If the question being asked is not whether a chemical induces gene mutations but, rather, whether the exposure received by humans causes any important risk from gene mutations, a much smaller sample size may be acceptable, under certain conditions.A comparison of the relative efficiencies of the specific-locus test (for gene mutations and small deficiencies) and the heritable-translocation test (for transmissible chromosome rearrangements), in detecting the same proportional increases over the spontaneous frequencies of their respective types of genetic damage, shows that less work is involved in reaching a conclusive result in the specific-locus test. Proposed specific-locus tests using biochemical markers are at a considerable statistical disadvantage compared with the standard test (using 7 visible markers) for which there is available a very large historical control showing a very low mutation rate.