A standard-curve-free ΔΔCt method for rapid GMO compliance screening relative to labeling thresholds

Regulatory thresholds for genetically modified organism (GMO) content are widely adopted worldwide for labeling purposes. Routine inspection primarily requires determining whether GMO levels exceed, fall below, or approach the threshold, rather than requiring precise quantification. Derived from the comparative Ct method, the ΔΔCt method enables rapid GMO screening using threshold-matched quality control (QC) samples. In this study, we established a universal ΔΔCt cutoff of ±0.4 as a decision criterion for categorizing samples into three groups relative to the labeling limit. The robustness of this criterion was validated through an inter-laboratory collaborative study involving eight testing laboratories, two GM crop matrices (soybean and maize), and a broad range of GMO concentrations. This method eliminates standard curve preparation while supporting both rapid threshold-based screening and semi-quantitative estimation of GMO content. This dual functionality makes it particularly suitable for high-throughput regulatory testing. Overall, this cost-effective and technically robust workflow provides a practical tool to facilitate the implementation of GMO labeling regulations.