Defining the Interindividual Uncertainty Factor (UFH): Implications for Non-Cancer Dose Response Modeling. P. S. Price, R. E. Keenan, H. Carlson-Lynch, and C. Gillis, ChemRisk a Division of McLaren/Hart, 1685 Congress St., Portland, ME 04102
The inter-individual uncertainty factor (UFH) is one of the two original factors use in setting "protective levels" such as RfDs, ADIs, and MRLs. The characterizing this "factor" is fraught with difficulties. Discussions in the publish literature define the factor in vague and conflicting fashions. The factor suffers from conceptual ambiguities and definitions of the factor have significant ethical and regulatory policy implications. A number of technical issues also complicate the use of empirical data in the characterization of the factor. Finally, the ambiguity in the definition of interspecies factor affects the UFH. The inability to describe the UFH in an objective fashion has been highlighted in recent efforts to investigate the uncertainty and variation in the RfD and in dose response assessment (Swartout et al., 1997; Baird et al. 1996). UFH related concepts must be dealt with in the dose response modeling for non-carcinogenic responses. For example, Price et al. 1997 uses estimates of inter-chemical variation in UFH to define dose responses above the RfD. In this session Dr. Baird proposes an adjustment of the dose response observed in animals in a fashion that parallels the UFH. This presentation discusses the issues related to the RfD in a systematic fashion and suggests several principles for defining the factor.