Construct validity of the ASAS health index in psoriatic arthritis: a cross-sectional analysis.

The Assessment of SpondyloArthritis international Society health index (ASAS-HI) was designed to assess the global health of patients with spondyloarthritis, but its performance in psoriatic arthritis (PsA) is hardly known. We addressed the clinimetric properties of this instrument in patients with PsA. This was a cross-sectional observational study that included 90 consecutive patients with PsA. The measurement properties of ASAS-HI were analysed against the Disease Activity index for PSoriatic Arthritis (DAPSA) and the Psoriatic Arthritis Impact of Disease (PsAID) questionnaire. A multivariate analysis was performed to weigh the ASAS-HI items associated with DAPSA active disease and PsAID high impact. Mean ASAS-HI was 5.8 (4.3). Convergent validity was high both against DAPSA (ρ 0.78, P < 0.0001) and PsAID (ρ 0.80, P < 0.0001). ASAS-HI showed a high discriminant capacity for both DAPSA remission [optimal criterion ≤ 2, area under the receiver operating characteristic curve (AUC) 0.92 (95% CI: 0.85, 0.97), P < 0.0001], and low activity [optimal criterion ≤6, AUC 0.87 (95% CI: 0.79, 0.94), P < 0.0001]. The ASAS-HI items significantly associated with DAPSA active disease were: 'I find it hard to stand for long' (β 4.48, P < 0.0001), 'I find it hard to concentrate' (β 2.94, P = 0.042) and 'I sleep badly at night' (β 1.86, P = 0.044). As for PsAID, the only item significantly associated with a high impact was 'I sleep badly at night' (β -3.29, P = 0.015). We demonstrated construct validity of ASAS-HI, a spondyloarthritis instrument, for the assessment of global health in patients with PsA.

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