Maarten Cozijnsen

98 Methods We performed a nationwide, observational cohort study and invited all Dutch pediatric gastroenterologists that prescribe biologicals to pediatric IBD patients, to participate. Those who agreed were asked to identify all eligible patients. Inclusion criteria were: (1) diagnosed with CD; (2) treated with adalimumab before the age of 18; and (3) treated with infliximab before the start of adalimumab. Exclusion criteria were: (1) conflicting comorbidity (such as auto-immune diseases or other chronic intestinal pathology); (2) recorded bad treatment adherence; and (3) previous participation in the prospective study by Hyams et al.(12) We collected the following information from patient records: patient characteristics, disease localization and behavior, previous treatment history, longitudinal disease activity from the start of adalimumab treatment and adverse events. Adalimumab efficacy assessment In order to assess the efficacy of adalimumab therapy, disease activity was retrospectively assessed by the first or second author using the mathematically weighted pediatric Crohn’s disease activity index (wPCDAI)(15) or a 4-stepped Physician Global Assessment (PGA) when the wPCDAI could not be calculated. Disease activity was categorized into 4 categories: remission, mild, moderate, and severe. Since timing of follow-up visits was unstructured, follow-up visits were sought closest to and pooled at the following time-points: 1 month, 4 months, 8 months and 12 months after the start of treatment and then yearly depending on availability. Based on the development in disease activity, the effect of adalimumab at each time point was categorized as either clinical response, clinical remission, no response or loss of response. Clinical response was defined as a decrease in the wPCDAI by at least 17.5 points, or a decrease in PGA from either moderate or severe to mild. A wPCDAI below 12.5 or a PGA equaling zero was defined as remission. Loss of response was defined as an increased disease activity to moderate or severe after having reached response or remission. If adalimumab treatment was discontinued, we documented the reason for discontinuation. In the case of lack of efficacy (non-response, loss of response) or intolerance (allergic reaction or adverse events), we considered these patients as having failed adalimumab. Secondly, we considered patients requiring CD related surgery during follow-up as having failed adalimumab due to loss of efficacy. Follow-up of