How normalization of reporter gene in vitro assays affects the results

Normalization strategies in reporter gene in vitro assays may influence the interpretation of chemical bioactivity, yet their implications are rarely systematically assessed. This study evaluates how different normalization approaches-subtraction versus division by baseline controls and normalization to positive controls-affect assay outcomes across three receptor-based systems: aryl hydrocarbon receptor, estrogen receptor agonist, and nuclear factor erythroid 2-related factor 2. Analyses are done on >400 wastewater samples tested in 34 to 54 microplates, including reference chemical concentration-response curves, 1% methanol, 1% ethanol, evaporated solvents, and media controls. Results demonstrate that even low solvent concentrations can significantly alter receptor responses. Evaporation of solvents mitigated adverse effects but may introduce biases due to loss of volatile or hydrophobic compounds in samples. Variability in assay signals was predominantly driven by systematic factors such as cell growth and reader sensitivity rather than random noise. Normalization to baseline and positive controls consistently yielded the most reproducible effect concentration values across plates and campaigns. The findings underscore the importance of rigorous control validation and normalization within in vitro bioassays, particularly when applied to complex environmental samples. Recommendations include minimizing solvent concentrations, validating solvent effects per assay, and using dual normalization strategies. The study also highlights the need for further research into time-dependent toxicity dynamics and signal quenching by environmental matrices to enhance in vitro assay robustness and comparability.