Els van de Vijver

43 but were blinded to the results of the calgranulin-C test. In case patients were assigned to the alternative reference standard, they were re-evaluated using the IBD Risk Stratifier until six months after inclusion. In case the initial risk stratum changed to ‘high-risk’, endoscopy was performed ultimately. Statistical analysis Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS, version 22.0 for Windows) and presented with GraphPad Prism (version 5 for Windows (San Diego, California, USA)). Diagnostic accuracy measures (sensitivity, specificity, positive predictive value, negative predictive value) were calculated for both the high-risk and low- risk stratum using predefined thresholds, as well as optimal thresholds (defined as the most upper left data point in the receiver operating characteristic (ROC) curve). Since we used both endoscopy and clinical follow-up as reference standard -with the latter being less accurate-, we used a Bayesian correction method to adjust for differential verification bias.(15-17) This method takes into account the verification pattern as well as bias due to imperfection of the clinical follow-up in a single model. The method requires specifying the verification pattern and giving a best guess of the accuracy of both reference standards in the form of a prior distribution. We assumed that endoscopy had 95–100% sensitivity and 95–100% specificity to diagnose IBD, and that clinical follow-up had a sensitivity of 80–100% and a specificity of 60–80% to diagnose latent IBD. Our inferences are based on the posterior distributions calculated using JAGS (‘Just Another Gibbs Sampler’), a free program licensed under GNU General Public License.(18) The R-package script is provided in supplementary data 1. The sample size calculation was previously described.(12) Human Subjects Protection This study was performed according to the Declaration of Helsinki. This study was conducted with the approval of the Medical Ethical Committee of the University Medical Center in Groningen (METc 2013/503) and Antwerp University Hospital (14/40/407). All participants aged 12 and above and their legal guardians gave informed consent to use data generated by routine medical care. The data were collected and recorded by the investigators in such a manner that subjects could not be identified, directly or through identifiers linked to the subjects. Head-to-head comparison Calgranulin C and Calprotectin 4343