Psychometric properties of the Rheumatoid Arthritis Disease Activity Index (RADAI) in a cohort of consecutive Dutch patients with RA starting anti-tumour necrosis factor treatment.

OBJECTIVES: To examine the psychometric properties of the self-administered Dutch Rheumatoid Arthritis Disease Activity Index (RADAI) and its short form (RADAI-SF) in patients with rheumatoid arthritis starting anti-tumour necrosis factor treatment.

METHOD: Internal consistency was assessed with Cronbach's alpha. A confirmatory factor analysis (CFA) was carried out to test the single-factor structure. Construct validity was examined by correlating RADAI and RADAI-SF scores with Disease Activity Score in 28 joints (DAS28). Internal responsiveness was evaluated with the paired t test and the standardised response mean (SRM). External responsiveness was assessed with receiver operating characteristic analysis and the SRM, using the EULAR response criterion as external criterion. Change scores were correlated with changes in DAS28.

RESULTS: At baseline and after 3 months' treatment, respectively, 191 and 171 patients completed the RADAI. The internal consistency of the RADAI and the RADAI-SF was satisfactory. CFAs confirmed the single-factor structure of both RADAI versions, but the short form provided the best model fit. Moderate correlations were found with the DAS28. SRMs of the RADAI and the RADAI-SF were, respectively, 0.76 and 0.80. Both versions had moderate accuracy to distinguish responders from non-responders. Changes scores were moderately correlated with DAS28 change scores.

CONCLUSIONS: This study showed satisfactory psychometric properties of the Dutch version of the RADAI. Omission of the tender joint count (RADAI-SF) produced comparable results and is justified for research purposes. The tender joint count might be useful as additional clinical information in patient management.