A straightforward composite score that adds a single CRP value to the CDAI four-variable framework. The result is a linear sum requiring no logarithms or square roots, making it one of the most transparent and auditable disease activity scores available.
The SDAI sums five variables in a simple linear equation: tender joint count, swollen joint count, physician global assessment (0-10), patient global assessment (0-10), and CRP in mg/dL. No logarithms, no square roots. The maximum possible score is 86.
The SDAI was the parent score from which the CDAI was derived. Adding CRP gives it a slight advantage over the CDAI in patients with clear serologic inflammation, but the two scores differ only by the CRP term and usually place a patient in the same category.
| SDAI Score | Activity State | Clinical Meaning |
|---|---|---|
| 3.3 or lower | Remission | Treat-to-target goal. Remission is associated with less radiographic progression, but progression can still occur. |
| above 3.3 to 11 | Low Activity | Acceptable if remission is not achievable. Monitor closely. |
| above 11 to 26 | Moderate Activity | Consider therapy adjustment. Escalation is usually considered if activity persists despite conventional DMARDs. |
| > 26 | High Activity | Active disease requiring treatment escalation. |
The EULAR 2022 treat-to-target goal is sustained remission or, where remission is not feasible, low disease activity. The SDAI defines these states as remission at 3.3 or lower and low disease activity above 3.3 to 11.
| Checkpoint | What to expect | If not met |
|---|---|---|
| Monitoring | Assess every 1 to 3 months while disease is active. Once the target is sustained, monitoring can be less frequent. | Not applicable |
| 3 months | At least 50% improvement in the composite disease activity measure. | Adjust therapy when there is no meaningful improvement. |
| 6 months | Remission in early disease, or at least low disease activity in established disease. | Escalate therapy. |
The CDAI, with remission at 2.8 or lower, is an alternative that needs no CRP value. It is an alternative when CRP is not available.
Source: Smolen JS, Landewe RBM, Bergstra SA, et al. EULAR recommendations for the management of rheumatoid arthritis with synthetic and biological disease-modifying antirheumatic drugs: 2022 update. Ann Rheum Dis. 2023. PMID 36357155. The 2025 EULAR update (published 2026) restates these recommendations; check the current version before publication.
The SDAI was developed by Smolen, Breedveld, and colleagues, published in Rheumatology in 2003. It was designed explicitly to be simple and calculable mentally or on paper without electronic assistance, which was a practical priority in the early 2000s. Aletaha and Smolen (2005) compared the construct, content, and discriminant validity of the SDAI, CDAI, and DAS28.
SDAI ranges from 0 to 86. A score of 3.3 or lower is remission, above 3.3 to 11 is low disease activity, above 11 to 26 is moderate, and above 26 is high. These cut-offs came from expert classification of 35 rheumatologists rating patient profiles, not from an outcome trial (Aletaha et al., 2005).
Both use the 28-joint tender and swollen counts and two global assessments. SDAI adds CRP in mg/dL, so it also reflects inflammation. CDAI leaves out laboratory values and uses different cut-offs, with remission at 2.8 or lower. One widely used approach is SDAI when acute-phase reactants are raised and CDAI otherwise (Mayo Clin Proc, 2012).
Milligrams per deciliter (mg/dL). If your lab reports CRP in mg/L, divide by 10 first. Entering the mg/L value directly makes the CRP contribution ten times too large.
It is a treat-to-target measure. The aim is remission or low disease activity, and the score helps show whether therapy should change. Comorbidities, medication safety, and the patient's own priorities still shape the decision.
Sources
Aletaha D, Ward MM, Machold KP, et al. Remission and active disease in rheumatoid arthritis: defining criteria for disease activity states. Arthritis Rheum. 2005;52(9):2625-2636.
Smolen JS, Breedveld FC, Schiff MH, et al. A simplified disease activity index for rheumatoid arthritis for use in clinical practice. Rheumatology. 2003;42(2):244-257.
Mayo Clin Proc. 2012;87(7):659-673. doi:10.1016/j.mayocp.2012.03.011 (composite disease activity measurement, Table 2).