How the CDAI (Crohn's Disease Activity Index) score is calculated
The Crohn's Disease Activity Index, often called the "Best index" after Rederick Best, was developed in 1976 for the National Cooperative Crohn's Disease Study and remains the most widely used clinical-trial measure of Crohn's disease activity. It combines a patient-completed 7-day symptom diary with a small set of clinical exam findings and lab values into a single weighted score. This calculator applies the original published formula exactly as defined.
The 8 weighted variables (Best index)
CDAI = 2x1 + 5x2 + 7x3 + 20x4 + 30x5 + 10x6 + 6x7 + 1x8, where each variable is:
- x1 — number of liquid or very soft stools, summed over 7 days.
- x2 — sum of daily abdominal pain ratings over 7 days (0 = none, 1 = mild, 2 = moderate, 3 = severe).
- x3 — sum of daily general well-being ratings over 7 days (0 = generally well, 1 = slightly under par, 2 = poor, 3 = very poor, 4 = terrible).
- x4 — number of listed complications currently present, out of 6 (arthritis/arthralgia; iritis/uveitis; erythema nodosum, pyoderma gangrenosum, or aphthous stomatitis; anal fissure/fistula/abscess; other fistula; fever over 100°F/37.8°C that week).
- x5 — taking Lomotil or opiates for diarrhea (0 = no, 1 = yes).
- x6 — abdominal mass on exam (0 = none, 2 = questionable, 5 = definite).
- x7 — hematocrit deficit: 47 minus hematocrit for men, or 42 minus hematocrit for women (can be negative if hematocrit is above the reference value, which lowers the score).
- x8 — percentage deviation from ideal body weight: 100 × (1 − actual weight ÷ ideal weight).
Interpreting your CDAI score
By convention, a CDAI under 150 indicates clinical remission, 150–220 indicates mildly active disease, 220–450 indicates moderately active disease, and above 450 indicates severe disease. In clinical trials, a "response" to treatment is often defined as a drop of 70–100 points from baseline, while remission is defined as a score under 150 regardless of the starting value.
Limitations and when to talk to your gastroenterologist
The CDAI depends on an honest, carefully kept 7-day symptom diary and an accurate clinical exam — it was designed for research and structured monitoring, not for self-diagnosis. It does not directly measure inflammation (unlike endoscopic scores or fecal calprotectin) and can be elevated by irritable-bowel-type symptoms even when inflammation is quiet. Use this calculator as an educational reference to track your own trend over time, and discuss any score — especially one in the moderate or severe range, or any meaningful change from your last calculation — with your gastroenterologist before changing treatment.