At the start of our trial we defined a sample size calculation, based on the previously described outcome measure including only patients with endoscopy. Based on a previous cohort study we expected that 46% of the recruited patients would undergo endoscopy. Using a sample size calculation based on independent samples calculated with a Fisher's exact test, we calculated that with 154 patients subjected to endoscopy the study would have 80% power to detect a 50% relative reduction of the primary outcome from 22% false positives with FC to 11% false positives with S100A12, at a one-sided alpha level of 0.05. The total number of patients to be recruited for this diagnostic accuracy study was therefore calculated at 335.