Laboratory assays
U-TXM in previously frozen aliquots of the urine samples was assayed in duplicate using a competitive ELISA (AspirinWorks® test kit, Corgenix, Peterborough, UK) as used in several previous studies [Assay values below or above the linear range after possible dilutions were imputed with the respective assay limit. The Pearson correlation between the U-TXM duplicate values where both were present was very high (0.98). The 2 participants with a baseline level in both duplicate measurements above the assay range after twofold dilution were excluded as estimation of any reduction would not be accurate. Imputed follow-up values (13 participants had both duplicate measurements imputed) were not excluded as this would cause bias (with high values more likely on placebo and low values on aspirin). The average of the U-TXM values across the duplicates (after imputation) divided by the creatinine was used as the sample result.