Els van de Vijver

103 IMPACT-III questionnaire The IMPACT-III questionnaire is a disease-specific quality-of-life questionnaire, that comprises 35 items in 6 domains: IBD-related symptoms (7 items), systemic symptoms (3), emotional functioning (7), social functioning (12), body image (3) and treatment/intervention-related concerns (3).(27) Each item is scored on a 5-point Likert scale, coded from 1 to 5 points. The maximum score is 175, higher scores indicate better quality-of-life. The Impact-III (NL) is a translated and modified version of the original Canadian Impact questionnaire (77) and has been validated for use in children of 8 years and older.(37) Laboratory tests Anaemia was defined as a haemoglobin (Hb) concentration more than two standard deviations below the mean of similarly aged children and adolescents from an iron supplemented USA reference population.(18) Hb Z-scores were calculated by subtracting the reference population mean (  ) from the individual score (x) and then dividing the difference by the reference population standard deviation (  ): = − . Iron deficiency was defined as a ferritin concentration below 30 µg/L.(19) Iron deficiency anaemia was defined as a combination of Hb < -2 Z-scores in combination with ferritin concentration < 30 µg/L. Intestinal inflammation was assessed by measuring faecal calprotectin. We used three commonly used categories: <250 μg/g for disease remission, >500 μg/g for disease flare, and 250-500 μg/g for the indecisive range.(29) 6-minute walking test Exercise capacity was assessed with the 6-minute walking test, and was expressed as the distance (in meters) a person can walk at a constant, uninterrupted pace in 6 minutes.(30) The 6-minute walking distance (6MWD) is age and sex dependent.(30, 31) Z-scores were calculated by subtracting the reference population mean (  ) from the individual score (x) and then dividing the difference by the reference population standard deviation (  ).(32) Self-reported fatigue and biological and functional parameters 103