Validity and reliability of the Short Form 36 Health Surveys (SF-36) among patients with spondyloarthritis in Singapore

The Short Form 36 Health Survey (SF-36) is a popular health-related quality of life (HrQoL) tool. However, few studies have assessed its psychometric properties in patients with spondyloarthritis (SpA). We therefore aimed to assess the reliability and validity of the SF-36 in patients with SpA in Singapore. Cross-sectional data from a registry of 196 SpA patients recruited from a dedicated tertiary referral clinic in Singapore from 2011 to 2014 was used. Analyses were guided by the COnsensus-based Standards for the selection of health Measurement INstruments framework. Internal consistency reliability was assessed using Cronbach's alpha. Construct validity was assessed through 33 a priori hypotheses by correlations of the eight subscales and two summary scores of SF-36 with other health outcomes. Known-group construct validity was assessed by comparison of the means of the subscales and summary scores of the SF-36 of SpA patients and the general population of Singapore using student's t tests. Among 196 patients (155 males (79.0 %), median (range) age: 36 (17-70), 166 Chinese (84.6 %)), SF-36 scales showed high internal consistency ranging from 0.88 to 0.90. Convergent construct validity was supported as shown by fulfillment of all hypotheses. Divergent construct validity was supported, as SF-36 MCS was not associated with PGA, pain and HAQ. Known-group construct validity showed SpA patients had lower scores of 3.8-12.5 when compared to the general population at p < 0.001. This study supports the SF-36 as a valid and reliable measure of HrQoL for use in patients with SpA at a single time point.