CDAI Calculator for Rheumatoid Arthritis
CDAI Calculator for Rheumatoid Arthritis
The rheumatology Clinical Disease Activity Index: two joint counts and two global assessments, added with equal weight and no laboratory value at all. Not the Crohn’s index of the same abbreviation.
CDAI (rheumatoid arthritis)
4 inputs → unweighted sumTender joint count 6, swollen joint count 4, patient global 4.5, evaluator global 3.5
Four terms, equal weight, no laboratory value
Remission ≤ 2.8 · low ≤ 10 · moderate ≤ 22 · high > 22
- which CDAI this is
- the rheumatology Clinical Disease Activity Index for rheumatoid arthritis. It is not the Crohn’s Disease Activity Index, which shares the abbreviation and nothing else: that instrument weights eight items over a seven-day symptom diary and its remission threshold is 150, not 2.8
- no acute-phase reactant
- this is the whole purpose of the index and not an omission. Aletaha and Smolen describe the CDAI as the only composite index without an acute-phase response, so it can be computed at the bedside, in a telephone or video consultation, and in settings where a CRP is not available the same day. Add the CRP and it becomes the SDAI
- equal weights
- every term carries weight exactly one. The DAS28 weights its components (0.56, 0.28, 0.70 and 0.014 on the ESR version) and takes square roots of the joint counts; the CDAI does neither. That is why the two indices classify different patients into the same named band and why their cut-offs are completely different numbers
- the two global assessments
- on 0–10. Trials use a 0–100 mm visual analogue scale and divide by ten. The evaluator global is the term that appears in the CDAI and the SDAI and in neither DAS28 nor the Boolean remission definition
- range
- 0 to 76. Of those 76 points, 56 are joints and 20 are global assessments, so the joint counts dominate the scale even though no item is weighted
Worked example
Tender joint count 6, swollen joint count 4, patient global 4.5, evaluator global 3.5
6 + 4 = 10 from the joint counts
4.5 + 3.5 = 8.0 from the two global assessments
10 + 8.0 = 18.0 — moderate disease activity, because 18.0 falls above 10 and at or below 22
No laboratory value enters anywhere. The same four numbers with a CRP of 12 mg/L give an SDAI of 19.2, because 12 mg/L is 1.2 mg/dL
Change the patient global from 4.5 to 9 and the CDAI becomes 22.5 — high disease activity — with no change in a single counted joint. Both globals carry full weight and that is the pattern to recognise
Enter the patient global as the raw 45 mm instead of 4.5 and the CDAI becomes 58.5. The index has no way to detect the error, which is why the scale is stated on every input
CDAI and SDAI cut-offs are different numbers
| Activity state | CDAI | SDAI | What a score of 10.5 is |
|---|---|---|---|
| Remission | ≤ 2.8 | ≤ 3.3 | — |
| Low disease activity | > 2.8 to 10 | > 3.3 to 11 | low on the SDAI |
| Moderate disease activity | > 10 to 22 | > 11 to 26 | moderate on the CDAI |
| High disease activity | > 22 | > 26 | — |
Where the 76 points live
| Component | Maximum | Share of the scale |
|---|---|---|
| Tender joint count | 28 | 36.8% |
| Swollen joint count | 28 | 36.8% |
| Patient global assessment | 10 | 13.2% |
| Evaluator global assessment | 10 | 13.2% |
Which CDAI this is, and what dropping the laboratory value buys
Two unrelated instruments are called the CDAI. This one is the rheumatology Clinical Disease Activity Index for rheumatoid arthritis: four items, added with equal weight, maximum 76, remission at 2.8 or less. The other is the Crohn’s Disease Activity Index, which weights eight items recorded over a seven-day diary, runs to several hundred points and calls remission below 150. They share an abbreviation and nothing else — no component, no scale, no cut-off — so a reader who arrived here looking for the gastroenterology index should follow that link rather than read on.
The reason this index exists is the item it leaves out. Aletaha and Smolen’s review describes the CDAI as the only composite disease activity index with no acute-phase response, which is what makes it computable anywhere: at the bedside, in a telephone or video consultation, in a clinic whose CRP comes back the following week, and retrospectively from a note that recorded joint counts and global assessments but no bloods. Add the CRP in mg/dL and the same four items become the SDAI, whose cut-offs are 3.3, 11 and 26 rather than 2.8, 10 and 22. What is lost by dropping it is the one component of disease activity the patient and the assessor cannot see: a patient with few tender and swollen joints, modest global scores and a CRP of 60 mg/L scores the same CDAI as an identical patient with an undetectable CRP. What is gained is that no part of the score waits on a laboratory, and that a raised CRP from an intercurrent infection cannot inflate it.
The deeper difference is weighting. The DAS28-ESR takes square roots of its joint counts and multiplies each term by a fitted coefficient, so a swollen joint is worth less than a tender one and the inflammatory marker can supply half the total. The CDAI weights nothing and transforms nothing. Three quarters of its 76 available points are joint counts, so in the middle of the range the joints dominate by magnitude even without a coefficient — and at the bottom of the range the opposite holds, because with one tender and one swollen joint already contributing 2, CDAI remission leaves only 0.8 between the two global assessments. The two indices therefore disagree most in exactly the patients where it matters: those whose counted synovitis and acute-phase response point in different directions, and those near a remission threshold.
Both global assessments are entered on 0–10 here. Trials record them on a 0–100 mm visual analogue scale and divide by ten, and entering the millimetre figure unchanged adds tens of points to a score whose whole remission band is 2.8 wide — so the calculator refuses an entry above 10 rather than absorbing it. The evaluator global is the item worth noticing when definitions disagree: it is in the CDAI and the SDAI and it is in neither DAS28 nor the ACR/EULAR Boolean remission criteria, which is one of the two reasons those definitions classify different patients as being in remission. Boolean and index-based remission criteria computes all of them at once.
Frequently asked questions
Is this the Crohn’s disease CDAI?
No. This is the rheumatology Clinical Disease Activity Index for rheumatoid arthritis: four items summed with equal weight, maximum 76, remission at 2.8 or less. The Crohn’s Disease Activity Index is a different instrument with the same abbreviation — eight weighted items over a seven-day diary, with remission below 150 — and it has its own page on this site.
What are the CDAI cut-offs?
Remission 2.8 or less, low disease activity above 2.8 up to 10, moderate above 10 up to 22, high above 22. They are not the SDAI’s cut-offs of 3.3, 11 and 26, and carrying one set onto the other index misclassifies patients: a score of 10.5 is moderate activity on the CDAI and low activity on the SDAI.
Why does the CDAI not include CRP?
Because that is its purpose. It is the only composite disease activity index with no acute-phase response, so it can be computed at the bedside or in a remote consultation without waiting for a laboratory. Adding CRP in mg/dL turns the same four items into the SDAI.
Why does the CDAI disagree with the DAS28?
Because the DAS28 weights its components and takes square roots of the joint counts, and the CDAI does neither. In the DAS28 the swollen joint count carries the smallest clinical coefficient and the inflammatory marker can supply half the score; in the CDAI every item carries weight one. The two indices also have completely different cut-offs.
Do I enter the global assessments as millimetres or out of ten?
Out of ten. Trials use a 0–100 mm visual analogue scale and divide by ten before summing, so a mark at 45 mm is 4.5 here. Entering 45 would add 45 points to a score whose remission threshold is 2.8, so this calculator refuses any entry above 10.
Related calculators
References
- Aletaha D, Smolen J. The Simplified Disease Activity Index (SDAI) and the Clinical Disease Activity Index (CDAI): a review of their usefulness and validity in rheumatoid arthritis. Clin Exp Rheumatol. 2005;23(5 Suppl 39):S100–8.
- MSB11456 in participants with moderately to severely active rheumatoid arthritis — statistical analysis plan, NCT04512001. ClinicalTrials.gov document archive. (States the SDAI and CDAI definitions, the CRP unit as mg/dL, both full cut-off sets and the ACR response definition.)
- A study of LY3462817 in participants with rheumatoid arthritis — protocol, NCT04634253. ClinicalTrials.gov document archive. (Independently corroborates SDAI ≤ 3.3 and ≤ 11, CDAI ≤ 2.8, and states the SDAI CRP input range as 0.1 to 10.0 mg/dL.)
- Studenic P, Aletaha D, de Wit M, et al. American College of Rheumatology/EULAR remission criteria for rheumatoid arthritis: 2022 revision. Ann Rheum Dis. 2023;82(1):74–80.
- Felson DT, Smolen JS, Wells G, et al. American College of Rheumatology/European League Against Rheumatism provisional definition of remission in rheumatoid arthritis for clinical trials. Ann Rheum Dis. 2011;70(3):404–13.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/