IBD Activity Score Calculator

The Crohn's Disease Activity Index (CDAI) is the classic, decades-old composite score used in clinical trials and practice to grade Crohn's disease severity. It was developed in 1976 by Best and colleagues as part of the National Cooperative Crohn's Disease Study and remains the reference index most trials still measure against, from a week of symptom tracking plus a handful of exam and lab findings: liquid or very soft stool frequency, abdominal pain rating, general well-being, a count of extra-intestinal complications (arthritis, iritis, erythema nodosum, fistula, fever, or other), antidiarrheal drug use, an abdominal mass on exam, hematocrit deviation from a sex-specific normal value, and body weight deviation from the patient's standard or ideal weight. Falling hematocrit reliably raises the score, because each percentage point below the reference hematocrit (47 for males, 42 for females) adds a fixed multiple of points, and clinically significant anemia in IBD tends to move that deviation by several points at once -- though the well-being and pain ratings, each scored on their own multi-point clinical scale rather than a single lab number, can swing the total by a wider margin across their full range than hematocrit alone.

The weight component works in the opposite direction from what intuition might suggest at first glance: it isn't raw body weight that raises the score, but weight running below the patient's own ideal weight, since active Crohn's disease classically causes weight loss -- so this same weight-deviation term falls (lowering CDAI) as measured weight climbs toward or above the ideal figure, holding ideal weight fixed. Standard cutoffs classify a score under 150 as clinical remission, 150-219 as mild, 220-449 as moderate, and 450 or above as severe disease, with a drop of 70 or more points generally considered a clinical response and 100 or more a significant response. CDAI does not incorporate endoscopic findings or inflammatory markers like CRP or fecal calprotectin, and modern practice increasingly favors those objective measures alongside or instead of a purely symptom-based score like this one.