Rheumatoid Arthritis Disease Activity Assessment

Definition and Calculation

The Simplified Disease Activity Index (SDAI) is calculated as the sum of five components: 28-joint swollen joint count, 28-joint tender joint count, provider global assessment of disease activity, patient global assessment of disease activity, and C-reactive protein measurement 1, 2
The Clinical Disease Activity Index (CDAI) is calculated as the sum of four components: 28-joint swollen joint count, 28-joint tender joint count, provider global assessment of disease activity, and patient global assessment of disease activity 1, 2

SDAI uses a scale of 0-86 with the following disease activity categories: Remission (≤3.3), Low disease activity (>3.3 to ≤11.0), Moderate disease activity (>11.0 to ≤26), and High/severe disease activity (>26) 1, 3
CDAI uses a scale of 0-76 with the following disease activity categories: Remission (≤2.8), Low disease activity (>2.8 to 10.0), Moderate disease activity (>10.0 to 22.0), and High/severe disease activity (>22.0) 1, 3

The American College of Rheumatology recommends using SDAI and CDAI for assessing disease activity in clinical practice 1, 2
CDAI has the advantage of not requiring laboratory tests, making it immediately calculable at the point of care 4, 5
SDAI includes C-reactive protein as an objective inflammatory marker but requires waiting for laboratory results, which may delay calculation 3

SDAI and CDAI provide more stringent definitions of disease activity and remission compared to DAS28 4
Unlike DAS28, SDAI and CDAI use simple arithmetic addition, making them easier to calculate in clinical practice 1

Proper training in joint count assessment is essential for accurate calculation of SDAI and CDAI 3
These measures should be used as part of a comprehensive approach to RA management that includes assessment of function and structural damage 6, 7

The American College of Rheumatology recommends using validated composite disease activity measures, including SDAI, CDAI, or DAS28, to monitor rheumatoid arthritis, with selection based on practice resources and the need for laboratory results 8

The Clinical Disease Activity Index (CDAI) provides more stringent remission criteria compared to DAS28, and does not require laboratory tests, making it immediately calculable at point of care for real-time clinical decisions 8
The Disease Activity Score-28 (DAS28) is recommended by EULAR for assessing disease activity and treatment response, but has less conservative remission criteria than SDAI and CDAI 9
The Simplified Disease Activity Index (SDAI) is an ACR/EULAR-recommended remission definition for clinical trials, and provides objective inflammatory marker data when CRP is elevated 8

The treatment target should be sustained remission (SDAI ≤3.3 or CDAI ≤2.8) or low disease activity, with monitoring every 1-3 months until remission is achieved, and every 3-6 months once remission or low disease activity is achieved 9

Proper training in 28-joint count assessment is essential for accurate calculation of all three measures, and the SDAI and CDAI provide more stringent disease activity definitions compared to DAS28, making them superior for treatment decisions when aiming for true remission 8
The use of biologic agents targeting specific inflammatory cytokines may disproportionately deflate composite scores that include acute phase reactants, making CDAI particularly valuable when biologics have normalized inflammatory markers but clinical synovitis persists 8