Abstract
OBJECTIVES: Physical function is a core outcome in PsA. We examined the construct validity and responsiveness of three commonly used instruments to assess physical function in PsA: HAQ disability index (HAQ-DI), MultiDimensional HAQ (MDHAQ) and the Patient-Reported Outcomes Measurement Information System (PROMIS®) Global-10. METHODS: Between 2016 and 2019, patients with PsA were enrolled in the Psoriatic Arthritis Research Consortium longitudinal cohort study in the USA. Correlations were calculated at baseline and among change scores using Spearman's correlation coefficient. Standardized response means were calculated. Agreement with the 20% improvement cut-off was used to determine the potential effect of using MDHAQ or the PROMIS Global-10 physical health (GPH) subscore in place of HAQ-DI when assessing the ACR20. RESULTS: A total of 274 patients were included in the analysis. The mean age of patients was 49 years and 51% were male. At baseline, the mean HAQ-DI was 0.6 (s.d. 0.6; range 0-3), the mean MDHAQ was 1.8 (s.d. 1.6; range 0-10) and the mean GPH T-score was 43.4 (s.d. 9.3; range 0-100). All three instruments were strongly correlated at baseline (rho 0.75-0.85). Change scores were moderately correlated (rho 0.42-0.71). Among therapy initiators, the mean change between two visits in HAQ-DI, MDHAQ and GPH was -0.1 (s.d. 0.4), -0.2 (s.d. 1.2) and 2.5 (s.d. 6.1), respectively. The standardized response means were 0.18, 0.16 and 0.41, respectively. CONCLUSION: The three instruments tested are not directly interchangeable but have overall similar levels of responsiveness.