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.
Original Formula
Smolen JS, Breedveld FC, et al.
Rheumatology, 2003
1
Joint Counts
Tender Joint Count (TJC)
/ 28
Swollen Joint Count (SJC)
/ 28
28 joints assessed: bilateral shoulders, elbows, wrists, MCPs (x5), PIPs (x5), and knees. Hips, ankles, and MTPs are not included.
2
Physician Global Assessment
Overall disease activity (physician assessment)
5
Rate overall disease activity 0-10 based on your clinical assessment. 0 = no activity, 10 = maximum.
0 (No activity)10 (Maximum)
3
Patient Global Assessment
Overall disease activity (patient assessment)
5
Ask the patient to rate how active their arthritis has felt over the past week, 0-10. This is their own perception, not a pain rating.
0 (No activity)10 (Maximum)
4
C-Reactive Protein (CRP)
CRP Value
mg/dL
SDAI uses mg/dL, not mg/L. The SDAI formula uses CRP in mg/dL (normal less than 1.0 mg/dL). If your lab reports in mg/L, divide by 10 (e.g. 12 mg/L = 1.2 mg/dL). This is different from the DAS28-CRP which uses mg/L.
ð What This Calculator Does
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 are highly correlated and most treatment decisions will be the same using either instrument.
ð Interpreting the Score
SDAI Score
Activity State
Clinical Meaning
0 to 3.3
Remission
Treat-to-target goal. Low risk of radiographic progression.
3.3 to 11
Low Activity
Acceptable if remission is not achievable. Monitor closely.
11 to 26
Moderate Activity
Consider therapy adjustment. Common biologic eligibility range.
> 26
High Activity
Active disease requiring treatment escalation.
ðĄ Pearls and Pitfalls
â
The math is transparent. Because SDAI is a simple sum, you can quickly see which variable is driving the score. A high SDAI driven entirely by patient global with minimal joint involvement tells a different clinical story than the same score with 15 swollen joints.
â
Unit consistency matters. The SDAI formula was designed for CRP in mg/dL. This is the most common source of calculation errors when clinicians use their usual DAS28-CRP mg/L value. Always check which unit your lab uses before entering.
â
SDAI and DAS28 remission thresholds are not interchangeable. A patient in DAS28 remission below 2.6 is not necessarily in SDAI remission below 3.3. The instruments use different formulas and the proportion meeting each threshold differs in most clinical cohorts.
ðŽ Evidence
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. Validation studies demonstrated strong correlation with radiographic progression and function, comparable to DAS28.
View Referencesâū
1
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. Original SDAI development and validation.
2
Aletaha D, Smolen JS.The Simplified Disease Activity Index (SDAI) and the Clinical Disease Activity Index (CDAI). Clin Exp Rheumatol. 2005;23(Suppl 39):S100-108. Comprehensive review of SDAI and CDAI validity and clinical use.
For clinical decision support only. This calculator does not replace clinical judgment. The SDAI is a monitoring and classification tool for established RA, not a diagnostic instrument. Note that SDAI uses CRP in mg/dL; verify your lab's units before entry.