This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Measuring rheumatoid arthritis activity
Treating rheumatoid arthritis to a target, remission or low disease activity, needs a reproducible measure of activity at each visit. The CDAI and SDAI, developed by Josef Smolen and Daniel Aletaha, are simple sums that do not need a calculator for logarithms, unlike the DAS28.
The formulas
- CDAI = tender joints + swollen joints + patient global + evaluator global
- SDAI = CDAI + CRP (mg/dL)
Joint counts use the 28-joint count: shoulders, elbows, wrists, the 10 metacarpophalangeal joints, the 10 proximal interphalangeal joints of the hands and the knees. Global assessments are rated from 0 (very well, no activity) to 10 (very poor, extreme activity). CRP is often reported in mg/L; divide by 10 for the SDAI.
Example
6 tender joints, 4 swollen joints, a patient global of 5 and an evaluator global of 4 give a CDAI of 19: moderate activity. With a CRP of 12 mg/L (1.2 mg/dL), the SDAI is 20.2, also moderate.
| Activity | CDAI | SDAI |
|---|---|---|
| Remission | 2.8 or less | 3.3 or less |
| Low | 2.9 to 10 | 3.4 to 11 |
| Moderate | 10.1 to 22 | 11.1 to 26 |
| High | above 22 | above 26 |
CDAI, SDAI or DAS28?
- The CDAI can be calculated immediately during the visit, without waiting for blood results, which makes it practical for treat-to-target decisions.
- The SDAI adds CRP and is one of the definitions of remission accepted by the ACR and EULAR (SDAI of 3.3 or less).
- The DAS28 weights tender joints and inflammation markers differently and tends to call remission more often, especially with drugs that strongly lower CRP.
Good practice
Use the same examiner and the same method at each visit when possible. Pain from osteoarthritis or fibromyalgia can raise tender joint counts and global scores without inflammatory activity, so the result is always read alongside the clinical examination.