Els van de Vijver

101 INTRODUCTION Inflammatory bowel disease (IBD) is a chronic illness of the gastrointestinal tract characterized by episodes of inflammation and remission and has significant impact on psychological and social functioning.(1-3) Fatigue is a common feature during active inflammation as well as during disease remission, which further decreases quality-of-life and hinders participation in daily activities.(5) Fatigue is defined as a ‘subjectively overwhelming sense of tiredness, lack of energy, and feeling of exhaustion that decreases one’s capacity for physical and mental activity’.(4) Though fatigue is considered a patient- relevant outcome measure, (5) studies on fatigue are limited. We recently explored the paediatric literature about fatigue in IBD and concluded that biological, functional, as well as behavioral factors contribute to fatigue.(6) Several studies suggest that disease activity and anaemia in particular affect the physical functioning of paediatric IBD patients.(7-10) Disease activity and fatigue Persisting mucosal inflammation may go unnoticed when response to therapy is only monitored by clinical parameters such as the paediatric Crohn’s disease activity index (PCDAI). The decision to escalate therapy may then be seriously delayed with consequently lower exercise capacity, quality of life and fatigue. Elevated faecal calprotectin levels correlate with endoscopic active disease and thus better reflect ongoing disease activity than clinical parameters.(11) Anaemia and fatigue Anaemia is a common systemic complication in IBD and significantly impacts on physical performance, quality of life, and absenteeism from school and extracurricular activities.(12-14) In adult IBD patients normalisation of haemoglobin levels improves general well-being, physical ability and quality of life.(15) To improve our understanding of fatigue in paediatric IBD, we conducted a cross-sectional observational study to assess the relationship between biological and functional factors. We hypothesize that patients with IBD who have elevated faecal calprotectin levels (>250 ug/g) (16, 17), anaemia (Hb < -2SD) (18) or low iron stores (ferritin < 30 µg/L) (19) have a Self-reported fatigue and biological and functional parameters 101