Tender and Swollen Joint Counts Anchor Both Indices
Both SDAI and CDAI start from the same 28-joint tender and swollen counts.
- 28: Joint sites assessed (standard RA joint count)
- 56 pts: Max joint contribution (combined TJC+SJC scale)
- Yes: Shared by both indices (identical input data)
- ~5 min: Assessment time (brief bedside exam)
Counting tender and swollen joints
Each of 28 joints is checked for tenderness and swelling on exam.
Joint counts are the backbone shared by SDAI and CDAI alike.
Patient Global Assessment Adds the Subjective View
Patients rate their own disease activity on a simple 0-10 scale.
- 0-10 cm: Scale range (visual analog scale)
- Equal: Contribution weight (to physician global)
- Patient: Reported by (self-assessment)
- Both indices: Used in (SDAI and CDAI)
Why patient input matters
Patient-reported activity captures symptoms exams alone may miss.
Subjective global scores balance objective joint counts.
Physician Global Assessment Adds Clinical Judgment
The examining clinician independently rates overall activity 0-10.
- 0-10 cm: Scale range (clinician visual scale)
- Patient score: Independent of (separate rating)
- Physician: Reported by (clinical assessment)
- Both indices: Used in (SDAI and CDAI)
Clinical judgment as a data point
Physician global reflects trained interpretation of overall status.
Two global scores triangulate perceived disease burden.
CRP Is the One Term That Separates SDAI From CDAI
SDAI adds serum CRP (mg/L); CDAI omits labs entirely.
- <10 mg/L: CRP normal range (typical lab reference)
- SDAI only: Added to (CDAI stays lab-free)
- ~86: SDAI max score (components plus CRP)
- 76: CDAI max score (no lab component)
Why CDAI skips labs
CDAI was built for instant point-of-care scoring without blood draws.
Dropping CRP trades small precision for speed and access.
SDAI and CDAI Track Closely in Practice
Despite the lab difference, both indices usually classify patients alike.
- High: Typical agreement (category-level concordance)
- No lab wait: CDAI advantage (immediate result)
- CRP nuance: SDAI advantage (captures inflammation)
- ≤3.3: Remission cutoff (SDAI) (clinical threshold)
Choosing between the two
Use CDAI when labs are unavailable; SDAI when CRP is on hand.
Either index reliably tracks meaningful change in activity.