Healthcare & Clinical
CDAI & SDAI Rheumatoid Arthritis Disease Activity Calculator
Calculate ACR/EULAR Clinical Disease Activity Index CDAI = TJC28 + SJC28 + PtGA + PhGA (0–76) and Simplified Disease Activity Index SDAI = CDAI + CRP_mg_dL for treat-to-target RA remission.
Immediate clinic-visit RA treat-to-target scoring—with or without waiting for lab CRP!
Because DAS28 requires square roots, logarithms, and laboratory ESR/CRP turnaround—and because IL-6 receptor blockers (tocilizumab/sarilumab) normalize CRP pharmacologically even when synovitis persists—Josef S. Smolen and Daniel Aletaha (Arthritis Research & Therapy, 2005) introduced two linear, ACR/EULAR-endorsed indices: the lab-free Clinical Disease Activity Index CDAI = TJC28 + SJC28 + PtGA_0_10 + PhGA_0_10 (range 0–76) and the Simplified Disease Activity Index SDAI = CDAI + CRP_mg_dL (note CRP in mg/dL, i.e., mg/L divided by 10)!
Stringent ACR/EULAR Boolean-aligned cutoffs govern DMARD/biologic escalation at every rheumatology visit: CDAI <= 2.8 (SDAI <= 3.3) = Stringent Remission, > 2.8 to 10.0 (SDAI > 3.3 to 11.0) = Low Disease Activity, > 10.0 to 22.0 (SDAI > 11.0 to 26.0) = Moderate Activity, and > 22.0 (SDAI > 26.0) = High Disease Activity!
| RA Treat-to-Target State | CDAI Cutoff (0–76) | SDAI Cutoff (CDAI + CRP mg/dL) | DMARD / Biologic Action |
|---|---|---|---|
| ACR/EULAR Remission | <= 2.8 | <= 3.3 | Maintain Current Regimen; Consider Taper if Sustained |
| Low / Moderate Activity | 2.9 – 10.0 / 10.1 – 22.0 | 3.4 – 11.0 / 11.1 – 26.0 | Optimize MTX / Add or Switch bDMARD / tsDMARD |
| High Disease Activity | > 22.0 | > 26.0 | Escalate Therapy; Re-Evaluate in 1–3 Months |
Lab-Free Clinical Disease Activity Index (CDAI, 0 to 76)
Sums 28-joint tender count B2 (0–28), 28-joint swollen count B3 (0–28), Patient Global Assessment B4 (0–10 cm VAS), and Evaluator/Physician Global Assessment B5 (0–10 cm VAS).
=IF(OR(B2 < 0, B2 > 28, B3 < 0, B3 > 28, B4 < 0, B4 > 10, B5 < 0, B5 > 10), "Invalid Joint / VAS Range", ROUND(B2 + B3 + B4 + B5, 1))Simplified Disease Activity Index (SDAI = CDAI + CRP in mg/dL)
Adds C-reactive protein B6 (in mg/dL; divide mg/L by 10) to CDAI.
=IF(OR(B2 < 0, B2 > 28, B3 < 0, B3 > 28, B4 < 0, B4 > 10, B5 < 0, B5 > 10, B6 < 0), "Invalid Joint / CRP Inputs", ROUND(B2 + B3 + B4 + B5 + B6, 1))Try the example
Use the original values or change them to test your understanding.