Statistics
Relative quantification was made by expressing the molar amount of each individual lipid as a percentage of the sum of lipids analyzed in the sample. All samples were analyzed in duplicate injections and the average was used. Relative standard deviation for duplicates had an average of 2.4% (range 0–14%) and QC samples were distributed throughout the run to assess method stability during the run. Data was log transformed and was provided as geometric mean and standard deviation. Repeated measures ANOVA was used for the comparison between visits. Multivariate analysis was made using the SIMCA software (SIMCA, Sartorius AG, Germany). Difference between sampling method were evaluated by Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA). The OPLS-DA model cross validation was based on a grouping by subject id, so that all data from one subject was removed from the model and then predicted by the model. The coefficients of variation (CV) for repeated PEx measurements were calculated separately for each lipid. For this, the standard deviation of all subjects was divided by the mean value for all subjects. As the CVs were not normally distributed, we provided their medians and IQR for all lipids in the results. A value below 10 can be considered as good. No correction for multiple testing was done due to the exploratory nature of our study.