Why use it

CDAI provides an objective, reproducible measure of CD activity beyond subjective clinical impression. It forms the regulatory basis for FDA approval of multiple biologics in CD. It enables monitoring of treatment response, assessment of remission, identification of relapse, and classification of disease severity for clinical trial eligibility.

About

The Crohn's Disease Activity Index (CDAI) is the gold-standard validated scoring system for quantifying the clinical severity of Crohn's disease (CD). Developed by Best et al. in 1976 from a multicenter National Cooperative Crohn's Disease Study (NCCDS) cohort of 187 patients, the CDAI integrates 8 weighted variables recorded over a 7-day observation period: stool frequency, abdominal pain, general well-being, number of systemic complications, antidiarrheal drug use, abdominal mass on examination, hematocrit, and body weight relative to standard. It is the primary endpoint used in most major CD clinical trials (including biologic therapy approvals: infliximab, adalimumab, ustekinumab). Remission is defined as CDAI < 150, and clinical response as a decrease ≥ 100 or ≥ 70 points from baseline.

When to use

Baseline disease activity assessment before starting or changing CD therapy
Monitoring response to medical therapy (induction and maintenance)
Clinical trial eligibility determination (active CD typically defined as CDAI ≥ 220 or ≥ 150)
Determining whether a patient is in clinical remission (CDAI < 150)
Quantifying disease relapse after surgery or biological therapy discontinuation

Formula

CDAI = (Variable 1 × 2) + (Variable 2 × 5) + (Variable 3 × 7) + (Variable 4 × 20) + (Variable 5 × 30) + (Variable 6 × 10) + (Variable 7 × 6) + (Variable 8 × 1) Where: Var 1 = Total number of liquid/soft stools over 7 days Var 2 = Sum of daily abdominal pain scores over 7 days (0=none, 1=mild, 2=moderate, 3=severe) Var 3 = Sum of daily general well-being scores over 7 days (0=well, 1=slightly unwell, 2=poor, 3=very poor, 4=terrible) Var 4 = Number of complications (arthritis, skin lesions, iritis, anal fissures/fistulae, other fistulae, fever >37.8°C) Var 5 = Antidiarrheal drug use this week (0=no, 1=yes) Var 6 = Abdominal mass (0=none, 2=questionable, 5=definite) Var 7 = Hematocrit deficit: males = 47 − HCT; females = 42 − HCT (use 0 if positive value; max deficit counted) Var 8 = Body weight: 100 × (1 − actual weight / standard weight), with floor at 0 < 150: Remission | 150–219: Mild | 220–450: Moderate | > 450: Severe

Pearls & pitfalls

The CDAI requires a 7-day symptom diary — the patient should record symptoms daily for 7 days before the clinic visit for maximum accuracy.
CDAI < 150 defines REMISSION in most clinical trials — this is the primary endpoint for FDA-approved biologics in CD.
A decrease in CDAI ≥ 100 points from baseline is the standard definition of CLINICAL RESPONSE; ≥ 70 points in some older trials.
Hematocrit (not hemoglobin) is used in the formula — use the measured HCT, not a calculated one.
Standard body weight is based on height and sex using standard reference tables — 1959 Metropolitan Life Insurance tables are traditionally used.
Variable 8 (body weight deficit) rarely contributes significantly unless the patient is markedly underweight (>10% below standard weight).
Abdominal mass must be examined in person — it cannot be assessed by history alone (0 = none, 2 = questionable/mild, 5 = definite).
The Harvey-Bradshaw Index (HBI) is a simplified version (5 items, single-day assessment) that correlates with CDAI and is easier to use in clinic without a 7-day diary.
Complications counted in variable 4: arthritis/arthralgia, mucocutaneous lesions, iritis/uveitis, anal fissures/fistulae/abscesses, other fistulae, fever >37.8°C in the past week.

Next steps

CDAI < 150 (Remission): Continue current maintenance therapy. Monitor for relapse. Consider therapy de-escalation only in sustained remission. CDAI 150–219 (Mild): Optimize 5-ASA/budesonide, ensure adherence, assess for infection (C. diff) and exclude stricture. CDAI 220–450 (Moderate): Consider corticosteroids for induction; add or optimize immunomodulator/biologic if steroid-dependent; ensure TB/hepatitis screening before biologics. CDAI > 450 (Severe): Hospitalization consideration; IV corticosteroids; urgent biologic therapy (infliximab); surgical consultation.

Evidence

Best WR et al. (Gastroenterology 1976) derived CDAI from 187 patients in the NCCDS. Validated extensively in subsequent clinical trials. Used as primary endpoint in pivotal trials for infliximab (ACCENT I), adalimumab (CHARM), certolizumab (PRECISE), ustekinumab (UNIFI/CERTIFI), risankizumab (ADVANCE/MOTIVATE). AGA and ECCO guidelines reference CDAI as the standard measure for CD clinical activity in trial and clinical settings.

References

Best WR et al. Development of a Crohn's disease activity index. National Cooperative Crohn's Disease Study. Gastroenterology. 1976;70(3):439-444. — link
Best WR et al. Rederivation and validation of the Crohn's disease activity index (CDAI). Gastroenterology. 1979;77(4 Pt 2):843-846. — link
Hanauer SB et al. Maintenance infliximab for Crohn's disease: the ACCENT I randomised trial. Lancet. 2002;359(9317):1541-1549. — link

Crohn's Disease Activity Index (CDAI)

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