A 90-dose LD50 (or ED50) and a chronicity factor are proposed to help communicate the results of test involving repeated doses of compounds. To determine the oral 90-dose LD50, groups of animals are fed appropriate dietary levels of a compound for 90 days and then held long enough for any sick survivors to die or recover. Food consumption is recorded and dosages are expressed as milligrams of compound per kilogram of body weight per day. Statistically, the 90-dose LD50 is determined in the same way as the 1-dose LD50 using logarithms of dosages and percent mortality expressed as probits. A ratio of the 1-dose LD50 and 90-dose LD50 of a compound is a measure of its cumulative effects and is termed the chronicity factor. The largest one found so far is more than 500 times the smallest found. These factors permit objective comparison of different classes of compounds but whether the smaller distinctions within a class are significant, remains to be learned. By plotting the logarithm of time in days necessary to kill half of each group of animals against the logarithms of the daily dosages necessary to produce this effect, one may predict the 90-dose LD50 of some compounds with acceptable accuracy, but only a much smaller, limiting value for other compounds.

contained in PB-280 889 titled Journal Articles on Toxicology. Group 14.