Validity and reliability of the Assessment of Spondyloarthritis International Society Health Index in English-speaking patients with axial spondyloarthritis in Singapore.

OBJECTIVES

To assess the validity and reliability of the Assessment of Spondyloarthritis International Society Health Index (ASAS HI) among patients with axial spondyloarthritis (axSpA) in Singapore.

METHODS

We collected data from English-speaking patients with axSpA seen at a dedicated axSpA clinic in a Singapore tertiary referral hospital from 2017 to 2018. Face validity of the English version of ASAS HI was assessed through cognitive debriefing interviews (CDIs). Structural validity was assessed with confirmatory factor analysis. Convergent construct validity was assessed with 12 a priori hypotheses about the magnitude and direction of correlations between the ASAS HI summary score and other patient-reported outcome measures. Internal consistency was assessed using Cronbach's alpha. Test-retest reliability was assessed by intraclass correlation coefficient (ICC). Measurement error was assessed by analyzing smallest detectable change (SDC).

RESULTS

Ten patients (age range 22-46 years, 50% male) participated in CDIs and face validity was supported. Among 108 patients (median age: 37 [21-77], 80.6% males), unidimensionality was confirmed (comparative fit index = 0.960, Tucker-Lewis Index = 0.952, root mean square error of approximation = 0.038, standardized root mean residuals = 0.068, model Chi-square test P = 0.1251) in the 17-item ASAS HI. The ASAS HI showed good internal consistency of 0.83 and excellent test-retest reliability (ICC = 0.95; 95% CI 0.91-0.98) when baseline was compared with week 2. SDC was 1.02. Convergent validity was supported as hypotheses were confirmed in 100% of the results.

CONCLUSIONS

This study supports the ASAS HI as a valid and reliable measure of health status for use in patients with axSpA in Singapore.