CDAI: Crohn's Disease Activity Index (1976)

The CDAI, or Crohn's Disease Activity Index, combines eight items into one number for how active Crohn's disease is. Three come from a 7-day patient diary (liquid stools, abdominal pain and general well-being). The rest are extra-intestinal findings, use of antidiarrhoeal drugs, an abdominal mass, haematocrit and body weight. Each item has a weight, and the weighted items are added. Magent's calculator applies the weights to the values you supply and returns the rounded index.

Not a medical device. Not a diagnosis or dosing recommendation.

Use this calculator via API

GI calculators
Best WR et al. · 1976
The CDAI formula
Worked examples
Frequently asked questions

What is the CDAI?

Investigators in the National Cooperative Crohn's Disease Study built the index in 1976. They asked physicians to rate overall disease activity, then found which measurable items, with which weights, most closely matched those ratings. The result has been used widely in Crohn's research.

The same letters, CDAI, also stand for the Clinical Disease Activity Index in rheumatoid arthritis. That is a different score with its own page: CDAI for rheumatoid arthritis.

The calculator returns the number. It doesn't sort it into activity bands.

The CDAI formula

CDAI =  2 x liquid or very soft stools (7-day total)
     +  5 x abdominal pain ratings (7-day sum, each day 0-3)
     +  7 x general well-being ratings (7-day sum, each day 0-4)
     + 20 x number of extra-intestinal findings (0-6)
     + 30 if taking diphenoxylate/atropine or opiates for diarrhoea
     + 10 x abdominal mass (0 none, 2 questionable, 5 definite)
     +  6 x haematocrit deficit (47 - Hct for males, 42 - Hct for females)
     +  1 x percent below standard weight

rounded to a whole number

Exact expression the calculator uses:

CDAI = liquid_stool_count_7d*2 + abdominal_pain_sum_7d*5 + wellbeing_sum_7d*7 + extra_intestinal_count*20 + taking_antidiarrheal*30 + abdominal_mass*10 + hematocrit_deficit*6 + weight_percent_deviation; hematocrit_deficit = 47-hematocrit_percent (male) or 42-hematocrit_percent (female); Best 1976; score rounded after the weight term

Inputs

Input	Meaning	Weight
`liquid_stool_count_7d`	Liquid or very soft stools over 7 days	×2
`abdominal_pain_sum_7d`	Sum of daily pain ratings, each 0 to 3 (0 to 21)	×5
`wellbeing_sum_7d`	Sum of daily well-being ratings, each 0 to 4 (0 to 28)	×7
`extra_intestinal_count`	Count of extra-intestinal findings (0 to 6)	×20
`taking_antidiarrheal`	Diphenoxylate/atropine or opiates for diarrhoea	30 if yes
`abdominal_mass`	0 none, 2 questionable, 5 definite	×10
`sex` and `hematocrit_percent`	Sets the haematocrit deficit	×6
`weight_percent_deviation`	Percent below standard weight, worked out by you	×1

The calculator takes the count of Best's six extra-intestinal findings (0 to 6); you count them.

How the calculation works

Diary items. Multiply the 7-day stool total by 2, the pain sum by 5 and the well-being sum by 7.
Findings. Multiply the extra-intestinal count by 20. Add 30 for antidiarrhoeal use. Multiply the mass code by 10.
Haematocrit. Subtract the haematocrit from 47 (males) or 42 (females) and multiply by 6. A haematocrit above the standard gives a negative term.
Weight. Add the percent below standard weight. Weight above standard gives a negative term.
Round. The total is rounded to a whole number.

Worked examples

These use Magent's own expected values for the calculator. Each changes one item from a baseline of zero, with a male haematocrit of 47 (no deficit).

Example 1: everything at zero

No diary points, no findings, haematocrit at the standard, weight at standard.
CDAI 0.

Example 2: five liquid stools in the week

5 × 2.
CDAI 10.

Example 3: well-being ratings adding to 4

4 × 7.
CDAI 28. In the same way, a pain sum of 3 gives 3 × 5 = 15.

Example 4: three extra-intestinal findings

3 × 20.
CDAI 60.

Example 5: a definite abdominal mass

Mass code 5 × 10.
CDAI 50. Antidiarrhoeal use alone would give 30.

Example 6: haematocrit 5 points below the standard

A female with haematocrit 37: 42 − 37 = 5, and 5 × 6 = 30.
CDAI 30. A male with haematocrit 42 gives the same, since 47 − 42 is also 5.

Example 7: weight 12% below standard

Weight term 12.
CDAI 12.

History of the CDAI

The study group published the index in Gastroenterology in 1976, in "Development of a Crohn's disease activity index. National Cooperative Crohn's Disease Study". The weights came from matching measurable items to physicians' overall ratings of activity. Activity bands often quoted with the index come from later trial use rather than from this calculator.

When it's used

The index is worked out after a 7-day diary has been kept, usually to describe Crohn's activity at a point in time or to compare it over time.

Where it's used

Gastroenterology clinics and, most of all, clinical trials in Crohn's disease.

Who uses it

Gastroenterologists, inflammatory bowel disease nurses and trial teams. The calculator applies the weights and adds the items.

Limits of the calculation

It needs a full 7-day diary. Missing days make the sums too low.
Several items are subjective, and the diary items carry much of the weight.
It doesn't measure inflammation directly, for example on endoscopy.
The calculator returns the number with no activity bands.
This page doesn't say what a result means for a particular person.

Clinical decisions belong to a qualified clinician who knows the patient.

Harvey-Bradshaw is a shorter Crohn's index based on one day, with no diary or blood test.
CDAI for rheumatoid arthritis shares the initials but scores joints in rheumatoid arthritis.
Mayo UC and PUCAI score ulcerative colitis, not Crohn's.

Glossary

Extra-intestinal findings: problems outside the bowel linked to Crohn's, such as joint or eye inflammation.
Haematocrit: the percentage of blood volume made up of red cells.
Standard weight: the reference weight the current weight is compared with.
Abdominal mass code: 0 none, 2 questionable, 5 definite.

Common mistakes

Using one day instead of seven. The diary items are 7-day totals.
Coding the mass as 1 to 3. The codes are 0, 2 and 5.
Using the wrong haematocrit standard. It's 47 for males and 42 for females.
Confusing it with the rheumatoid arthritis CDAI. Same initials, different score.

Source

Best WR, et al. Development of a Crohn's disease activity index. National Cooperative Crohn's Disease Study. Gastroenterology. 1976;70(3):439-444. DOI: 10.1016/S0016-5085(76)80163-1. PMID: 1248701

Frequently asked questions

What does CDAI stand for in Crohn's disease?

Crohn's Disease Activity Index.

Is this the same CDAI used for rheumatoid arthritis?

No. That's the Clinical Disease Activity Index, a different score. See CDAI for rheumatoid arthritis.

How many days of diary does the CDAI need?

Seven. Stools, pain and well-being are 7-day totals.

Can the haematocrit term be negative?

Yes. A haematocrit above 47 (males) or 42 (females) gives a negative term.

Does the calculator give activity bands?

No. It returns the rounded index.