Introduction:
To test the construct validity of the U9 ultrasonographic scale, to determine the cut-off points for different degrees of rheumatoid arthritis (RA) activity, and to determine whether or not US assessment with the U9 score is useful for monitoring the response to treatment of RA.

Material and methods:
A prospective, multicenter study was conducted in 4 different centers in Egypt. All RA patients who were recruited were subject to evaluation of clinical disease activity by the Clinical Disease Activity Index (CDAI) and Disease Activity Score of 28 joints based on erythrocyte sedimentation rate (DAS28-ESR). Assessment of the Functional Status by the Health Assessment Questionnaire (HAQ) and U9 ultrasound score was performed. All the targeted joints were assessed by EULAR recommendations and the combined score of EULAR/OMERACT (0–3). Targeted tendons scored 0–3. After three months of treatment, CDAI and DAS28-ESR, HAQ, and U9 were repeated to detect the response.

Results:
One hundred and forty patients with mean age 39.26 ±11.30 were recruited from 4 centers. With regard to convergent validity, the U9 ultrasonographic scale was significantly associated with clinical parameters (CDAI and DAS28-ESR) as well as functional state (HAQ) at both visits. Likewise, concerning discriminative validity, the U9 scale showed the ability to distinguish different grades of RA activity, presenting well-defined cut-off points of different grades (severe, moderate, and mild), with very good specificity and sensitivity (11.5, 5.5, and 3.5, respectively). A significant parallel decrease was detected in clinical and sonographic scales at the follow-up assessment.

Conclusions:
The U9 ultrasound scale showed good construct (convergent and discriminative) vali­dity and can be used to monitor the disease and therapeutic response to treatment in RA.