The CDAI or Clinical Disease Activity Index [1] and the SDAI or Simplified Disease Activity Index [2] are inspired by the « DAS » score family for Rheumatoid Arthritis, comprising namely DAS28 and DAS28-CRP. They are very useful to make an objective, reproducible and comparable assessment of the rheumatoid arthritis activity.

SDAI takes into account the following items:

• TJC28: The number of tender joints (0-28).
• SJC28: The number of swollen joints (0-28).
• CRP: The C-Reactive Protein concentration (in mg/dl, between 0 and 10).
• PaGH: The patient global health assessment (from 0=best to 10=worst).
• PrGH: The care provider global health assessment (from 0=best to 10=worst).

The computation of the score is done through the following equation:

$$\text{SDAI} = TJC28 + SJC28 + PaGH + PrGH + CRP$$

The interpretation of this score ranging from 0 to 86 does not depend on its evolution over time. Generally, remission is considered achieved if the score is between 0 and 3.3 included. Low activity corresponds to >3.3 to 11 included. Moderate activity is between >11 and 26 included, while high activity is strictly above 26.

Both for CDAI and SDAI, the 28 tender or swollen joint scores target the same joints (shoulders, elbows, wrists, metacarpophalangeal joints, proximal interphalangeal joints and the knees).