CDAI vs SDAI in Rheumatoid Arthritis Disease Activity Assessment
Key Difference: CDAI Does Not Require Laboratory Testing
The primary difference is that CDAI excludes C-reactive protein (CRP) and can be calculated immediately at the point of care, while SDAI includes CRP as an objective inflammatory marker but requires waiting for laboratory results. 1, 2
Calculation Components
SDAI (Scale 0-86)
- 28-joint tender count
- 28-joint swollen count
- Patient global assessment (0-10 cm VAS)
- Provider global assessment (0-10 cm VAS)
- CRP in mg/dL 2
CDAI (Scale 0-76)
- Same four clinical components as SDAI
- No laboratory test required 2
Both use simple arithmetic addition, making them easier to calculate than DAS28, which requires complex differential weighting. 3
Disease Activity Categories
SDAI cutpoints: Remission ≤3.3, Low >3.3 to ≤11, Moderate >11 to ≤26, High >26 1, 2
CDAI cutpoints: Remission ≤2.8, Low >2.8 to 10, Moderate >10 to 22, High >22 1, 2
Both provide more stringent remission definitions compared to DAS28 (≤2.4), meaning patients classified as remission by DAS28 may not meet SDAI/CDAI remission criteria. 1, 3
Clinical Application Algorithm
Use CDAI when:
- Immediate point-of-care decision making is needed 2
- Laboratory results are delayed or unavailable 1
- In smaller practice settings with logistical barriers 1
Use SDAI when:
- An objective inflammatory marker is desired for more comprehensive assessment 1
- Patients have elevated acute-phase reactants at baseline 1
- Laboratory infrastructure supports timely CRP measurement 4
The Mayo Clinic approach recommends using SDAI for patients with increased acute-phase reactant levels, and CDAI for all others. 1
Validation and Performance
Both SDAI and CDAI are recommended by the American College of Rheumatology for clinical practice and have equivalent validity. 1, 2 They correlate highly with DAS28, HAQ scores, and ACR response criteria. 5 Both demonstrate construct validity by associating with functional improvement and radiographic progression. 6
The remission criteria for CDAI and SDAI are currently the most stringent available. 7
Important Clinical Considerations
Training requirement: Proper training in 28-joint count assessment is essential for accurate calculation of both measures. 2
CRP reporting: When calculating SDAI, CRP must be in mg/dL. For values reported as <0.3 mg/dL (below detectable range), use 0.29 for calculation. 1
Biologic therapy effects: Newer biologic agents targeting specific inflammatory cytokines may disproportionately affect CRP levels, potentially making SDAI appear more favorable than clinical examination alone would suggest. 1
Comorbidity influence: Both measures can be affected by conditions causing tender joints (e.g., fibromyalgia) or elevated acute-phase reactants from non-RA causes. 1