SDAI (Simplified Disease Activity Index)
Disease activity in rheumatoid arthritis including CRP
- ACR/EULAR 2011 remission (SDAI ≤ 3.3)
- ACR 2019 recommended measure
Sources last checked: 2026-10-13
SDAI (Simplified Disease Activity Index)
Disease activity in rheumatoid arthritis including CRP
Entering CRP in mg/L instead of mg/dL inflates the SDAI tenfold for the CRP component — the commonest error with this index.
Interpretation
| ≤ 3.3 — remission | ACR/EULAR 2011 index-based remission definition. |
|---|---|
| > 3.3 to 11 — low disease activity | |
| > 11 to 26 — moderate disease activity | |
| > 26 — high disease activity |
What is the SDAI (Simplified Disease Activity Index)?
The Simplified Disease Activity Index (SDAI) adds tender and swollen joint counts (28 joints), patient and evaluator global assessments (0–10) and CRP in mg/dL. It was introduced by Smolen and colleagues in 2003 using data from leflunomide trials. SDAI ≤ 3.3 is one of the ACR/EULAR 2011 definitions of remission.
When to use it
- To assess rheumatoid arthritis activity when CRP is available
- To define remission in trials and practice (SDAI ≤ 3.3)
- In treat-to-target follow-up
When not to use it
- With CRP in mg/L — the index is defined in mg/dL
- On IL-6 blockade, where CRP is suppressed; the CDAI does not depend on CRP
How it is calculated
SDAI = TJC28 + SJC28 + patient global (0–10) + evaluator global (0–10) + CRP (mg/dL) CRP in mg/dL, not mg/L.
Clinical pearls and limitations
- Entering CRP in mg/L instead of mg/dL inflates the SDAI tenfold for the CRP component — the commonest error with this index.
- Divide CRP in mg/L by 10 before entering it.
- CRP contributes little to the total in most patients, which is why the CDAI performs similarly.
- SDAI remission closely matches the ACR/EULAR Boolean remission definition.
Frequently asked questions
What SDAI score is remission?
3.3 or less. Low disease activity is > 3.3 to 11, moderate > 11 to 26, high > 26.
Which CRP unit does the SDAI use?
mg/dL. A CRP of 5 mg/L is entered as 0.5.
What is the maximum SDAI?
86, assuming a CRP ceiling of 10 mg/dL.
Evidence and guidelines
- Primary source: Smolen JS, Breedveld FC, Schiff MH, et al. A simplified disease activity index for rheumatoid arthritis for use in clinical practice. Rheumatology (Oxford) 2003;42(2):244-257.
- England BR et al. 2019 Update of the ACR recommended RA disease activity measures. Arthritis Care Res 2019;71(12):1540-1555 (SDAI category cut-offs).
- Felson DT et al. ACR/EULAR provisional definition of remission in rheumatoid arthritis for clinical trials. Arthritis Rheum 2011;63(3):573-586 (SDAI ≤ 3.3).
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