SDAI and CDAI: Composite Disease Activity Measures for Rheumatoid Arthritis
The Simplified Disease Activity Index (SDAI) and Clinical Disease Activity Index (CDAI) are validated composite measures used to assess disease activity in rheumatoid arthritis patients, with CDAI being calculated as the sum of 28-joint swollen and tender counts plus provider and patient global assessments, while SDAI additionally includes CRP measurement.
Definition and Components
SDAI is calculated as the sum of five components: 28-joint swollen joint count (28SJC), 28-joint tender joint count (28TJC), provider global assessment of disease activity (PrGA, 0-10 scale), patient global assessment of disease activity (PtGA, 0-10 scale), and C-reactive protein (CRP, mg/dL) 1
CDAI is calculated as the sum of four components: 28SJC, 28TJC, PrGA (0-10 scale), and PtGA (0-10 scale), without requiring laboratory values 1
Both indices were developed to provide simple, practical tools for assessing rheumatoid arthritis disease activity in clinical practice 2
Scoring Ranges and Disease Activity Categories
SDAI uses a scale of 0-86 with the following disease activity categories 1:
- Remission: ≤3.3
- Low disease activity: >3.3 to ≤11.0
- Moderate disease activity: >11.0 to ≤26
- High/severe disease activity: >26
CDAI uses a scale of 0-76 with the following disease activity categories 1:
- Remission: ≤2.8
- Low disease activity: >2.8 to 10.0
- Moderate disease activity: >10.0 to 22.0
- High/severe disease activity: >22.0
Clinical Applications and Advantages
Both SDAI and CDAI are recommended by the American College of Rheumatology for assessing disease activity in clinical practice 1
CDAI has the advantage of not requiring laboratory tests, making it immediately calculable at the point of care 1
SDAI includes CRP as an objective inflammatory marker but requires waiting for laboratory results, which may delay calculation 1, 3
Both measures are more stringent in defining remission compared to other measures like DAS28, providing more accurate assessment of disease control 4
Regular assessment using these composite measures is recommended to guide treatment decisions in a treat-to-target approach 5
Comparison with Other Disease Activity Measures
SDAI and CDAI provide more stringent definitions of disease activity and remission compared to DAS28 1, 6
Unlike DAS28, which uses complex mathematical formulas including square roots and logarithms, SDAI and CDAI use simple arithmetic addition, making them easier to calculate in clinical practice 1, 7
Patient-driven tools (PAS, PAS-II, RAPID-3) do not require formal joint counts but lack the objective assessment provided by provider evaluation in CDAI and SDAI 1
DAS28 remission criteria are less stringent than SDAI/CDAI, with studies showing 74-77% of patients in DAS28(CRP) remission not achieving Boolean remission 6
Clinical Significance and Validation
Both indices have been validated as outcome measures for rheumatoid arthritis and correlate well with other established measures like DAS28 and HAQ scores 7
SDAI and CDAI enable assessment of actual disease activity, response to therapy, and achievement of remission 2, 4
The simplicity of these scores allows patients to better understand their disease activity level and correlate changes with their symptoms 2
Recent research has validated a quick quantitative CRP assay (SDAI-Q) that shows excellent agreement with conventional SDAI, potentially making SDAI more practical for immediate point-of-care use 3
Common Pitfalls and Caveats
Relying solely on these indices without considering other aspects of the disease may lead to incomplete assessment 5
Patient global assessment scores often limit patients from achieving Boolean remission even when they achieve SDAI or CDAI remission 6
These measures should be used as part of a comprehensive approach to RA management that includes assessment of function and structural damage 1
Proper training in joint count assessment is essential for accurate calculation of these indices 1