Objective To develop, validate and apply a generic clinical severity index applicable to all adult patients with inflammatory bowel disease (IBD).

Design A review of the literature and an expert focus group consultation were carried out in order to draw out relevant items from existing literature. The new index was called the IBD Index (IBDEX). Standard psychometric analysis was carried out. The construct validity was assessed against biochemical markers, clinical and endoscopic indices. The new index was completed again within 6 weeks to check responsiveness and reproducibility.

Results IBDEX was used to assess 255 adult patients with IBD (125 with Crohn’s disease and 130 with ulcerative colitis), and 64 patients were re-evaluated within 6 weeks. It had good internal consistency (Cronbach's α=0.79) and correlated very well with the Harvey Bradshaw Index (r=0.94), the Simple Clinical Colitis Activity Index (r=0.92), the Mayo Clinic Index (r=0.87) and the Simple Endoscopic Score (r=0.76), all with p values <0.05. IBDEX had a moderate but positive correlation with C reactive protein (r=0.51) and erythrocyte sedimentation rate (r=0.36) p values both <0.05. The test–retest reliability was good (intraclass correlation coefficient 0.97) and responsiveness ratio was 2.27.

Conclusions IBDEX is the first properly validated Clinical Disease Severity Index in IBD. Our results showed that it is valid, reliable and reproducible and has the potential to be used in clinical practice.