Four-parameter nonlinear regression and maximum achievable effect in ecotoxicology: just visually appealing or...

Area of Science:

• Ecotoxicology
• Environmental Science
• Chemical Risk Assessment

Background:

• The definition of ECx (concentration causing x% effect) is standard in ecotoxicology.
• Four-parameter regressions (4PR) allow for two ECx calculations: absolute and relative.
• Current software and literature lack uniformity in choosing between absolute and relative ECx.

Purpose of the Study:

• To analyze the differences between absolute and relative ECx values derived from 4PR.
• To highlight the impact of curve steepness and lower limit on ECx calculations.
• To emphasize the need for regulatory guidance on ECx determination.

Main Methods:

• Sensitivity analysis of 4PR models.
• Comparison of absolute ECx and relative ECx calculations.
• Literature review of current practices and software implementations.

Main Results:

• Relative ECx from 4PR can be significantly lower than absolute ECx.
• The choice between absolute and relative ECx depends on curve characteristics.
• Inconsistent application of ECx calculations is prevalent in current practice.

Conclusions:

• The selection of absolute versus relative ECx requires clear regulatory direction, not user preference.
• Standardized guidelines are crucial for consistent, transparent ecotoxicological data interpretation.
• Further discussion and detailed guidance are needed to address the ambiguity in ECx calculations from 4PR.