Discriminant validity of the Ankylosing Spondylitis Disease Activity Score (ASDAS) in patients with non-radiographic axial spondyloarthritis and ankylosing spondylitis: a cohort study

The aim of this study was to assess discriminant validity of Ankylosing Spondylitis Disease Activity Score (ASDAS)-C-reactive protein (-CRP) and ASDAS-erythrocyte sedimentation rate (-ESR) and to compare with The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) as clinical tools for the measurement of disease activity in patients with non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS). Also, the cut-off values for ASDAS-CRP in nr-axSpA and AS is revisited. Patients with axSpA were recruited from Erciyes Spondyloarthritis Cohort (ESPAC) and were assessed for disease activity, quality of life and functional measures. The discriminatory ability of ASDAS-CRP and ASDAS-ESR was assessed using standardized mean differences and receiver operating characteristic (ROC) curves analysis. Optimal cut-off values for disease activity scores were calculated. Two hundred and eighty-seven patients with axSpA (nr-axSpA:132, AS:155) were included in this study. Two ASDAS versions and BASDAI had good correlations with patient’s and physician’s global assessment in both groups. Discriminatory ability of ASDAS-CRP, ASDAS-ESR and BASDAI were similar in patients with nr-axSpA and AS when the patients were assigned into low and high disease activity according to the ASAS partial remission, patient’s and physician’s global assessment scores (based on the comparison of ROC curves). ASDAS cut-off values are quite similar between groups indicating that ASDAS-CRP works similarly well in nr-axSpA and AS. The performance of ASDAS to discriminate low and high disease activity and cut-off values are quite similar in patients with AS and non-radiographic axial SpA.