Cytotoxicity across OECD methods is not a single endpoint: context-dependent roles, variability and implications for decision-making.

Cytotoxicity and proliferation measurements are widely used in OECD in vitro test guidelines but do not represent a uniform endpoint. Their regulatory meaning depends on assay role, where they may function as classification endpoints, predictive surrogates, validity criteria, or constraints for concentration selection. These roles differ in biological interpretation, timing, and decision consequences. This review analyzes the context-dependent use of cytotoxicity, cytostasis, and proliferation metrics across OECD methods and emphasizes how endpoint biology, timing, and assay role determine the interpretation of reported viability-type values. Fixed cytotoxicity thresholds are applied as operational decision criteria but often overlap with assay variability, requiring uncertainty-aware interpretation. Excessive cytotoxicity may further compromise interpretability through non-specific stress and assay interference. Cytotoxicity should therefore be treated as a role-specific method-performance parameter requiring explicit definition of endpoint biology, timing, variability, and decision criteria. Good In Vitro Method Practices (GIVIMP)-aligned implementation would improve transparency, reproducibility, and regulatory interpretability of cytotoxicity-based decisions.