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Objective: To assess validity, responsiveness and interpretability of the UltraSound-CLinical ARthritis Activity (US-CLARA) index in patients with rheumatoid arthritis (RA). Methods: In this longitudinal study were involved RA patients starting treatment with abatacept. Subjects were followed along three visits in the first 6 months of therapy and underwent a comprehensive clinimetric evaluation. Validity was explored correlating the baseline scores and the cumulative inflammatory burden of the US-CLARA with the other composite indices applied. Sensitivity to change was assessed after 6 months of treatment in terms of internal and external responsiveness. Interpretability was defined in terms of determination of cutoffs against external criteria for remission (REM), low disease activity (LDA), moderate disease activity (MDA), and high disease activity (HDA) of SDAI. Results: One-hundred and thirty patients completed the study. Validity: moderate correlations were observed between US-CLARA and both DAS28-CRP and DAS28-ESR. Higher correlations were also found between US-CLARA and both SDAI and CDAI scores. Responsiveness: internal responsiveness was wide, with SRM and ES ranging from 0.91 to 1.51. US-CLARA responsiveness was similar to that of DAS28, SDAI, or CDAI. Similarly, the area under ROC curve (AUC-ROC) of US-CLARA gives identical results. The AUC of cumulative inflammatory burden, calculated during the 6-months follow-up of all combinations were highly correlated (p < 0.0001). Interpretability: cutoff values for REM, US-CLARA <2.0; for LDA, 2.0 ≤US-CLARA <3; for MDA, 3 ≤US-CLARA ≤4.8; for HDA, US-CLARA >4.8. Conclusion: The US-CLARA is valid and sensitive tool to assess disease activity in RA.