Discovery and Validation of Serum Metabolite Markers
The rank sum test was employed to search for low abundance discriminating ions. In the development cohort, there were 241 significantly changed ions in OSCC compared to the HC (FDR < 0.05). When the same procedure was conducted in the validation cohort, 218 ions were found to have significant differences. After overlapping the two batches of discriminating ions, 65 ions were confirmed to not only have statistical significance but also to become upregulated or downregulated in the same direction (Fig. 2A ). After removing ions that either were redundant or failed to meet the fold change criteria (larger than 1.5 or smaller than 0.67), 39 metabolites were finally selected as potential characteristic marker candidates (Table S3 ). A volcano graph highlighted the top 10 metabolites with the most obvious fold changes (Fig. 2B ), which shows that lipid molecules are the predominant species in the serum including glycerophosphocholine (GPC), lyso-glycerophosphocholine (Lyso-GPC), acyl carnitine, diacylglycerol (DG), sphingolipids (e.g., sphingosine-1-phosphate). Figure 2C displays the relative expressions of these top 10 metabolites in the form of a heatmap.