Performance of the Revised UDP
The revised OECD guideline 425 (OECD 2001a) has improved performance for prediction of the point estimate of lethality (LD50) and confidence intervals for chemicals with wide variability of response characteristics, even when the approximate LD50 and dose-response slope are not known. In addition, the revisions allow it to be used to evaluate lethality in the 2000 to 5000 mg/kg range for certain hazard classification purposes. The efficiencies of the new up-and-down dosing routine allow the LD50 to be estimated using relatively few animals. However, the guideline does not provide for determination of the slope of the dose-response curve. (When available from other acute tests, the slope is used to calculate other LDx values for use in human and environmental risk assessments and to improve assessment of the contribution of the substance to the toxicity of a mixture so that it is not necessary to test the mixture itself in animals.)