Fig. 4Reversed metabolite fold-change direction in discovery- and validation cohorts. a Score scatter plot for the initial OPLS-DA model of 4578 metabolite features in the discovery cohort (Fig. 1, step 2b). Blue dots represent PDAC samples (scores, n = 44); white dots represent CP samples (n = 23); t1 on the x-axis, first component; to1 on the y-axis, first of two orthogonal components. Model parameters: CV-groups, 7; R2X(cum) 0.39; Q2(cum) 0.22; CV-ANOVA p = 0.02. The model was used to select 259 features for further analysis. b Score scatter plot for the OPLS-DA model in the validation cohort based on the 19 metabolites remaining from the discovery cohort (Fig. 1, step 4b). Orange dots represent PDAC samples (n = 20); white dots represent CP samples (n = 31); to1 on the y-axis, first (and only) orthogonal component. Model parameters: CV-groups, 7; R2X(cum) 0.648; Q2(cum) 0.699; CV-ANOVA p = 1.7E-11. c Loading (metabolite) scatter plot for the discovery cohort OPLS-DA model. The phospholipids are situated to the right of the vertical line, close to the dummy CP variable, indicating that they are down-regulated in PDAC compared to CP. d Loading scatter plot for the validation cohort OPLS-DA model. The phospholipids are situated to the left of the vertical line, close to the dummy PDAC variable; hence they are up-regulated in PDAC compared to CP. Based on this reversal of their fold-change directions, the 14 phospholipids were excluded from the final metabolite marker panel