LAUSR

OBJECTIVE Assessing the construct validity of the Patient-Reported Outcomes Measurement Information System Physical Function 10-Item Short Form (PROMIS PF-10) in a subpopulation of rheumatoid arthritis (RA) or axial spondyloarthritis (axSpA) patients with severe limitations in physical functioning (PF). METHOD RA/axSpA patients with severe functional limitations completed the PROMIS PF-10, Health Assessment Questionnaire - Disability Index (HAQ-DI for RA) or Bath Ankylosing Spondylitis Functional Index (BASFI for axSpA), 36-item Short Form Health Survey (SF-36), EuroQol 5-dimensions 5-level (index score, EQ-VAS), and performed the Six-Minute Walk Test (6MWT). Construct validity was assessed by computing Spearman rank or Pearson correlation coefficients and testing hypotheses about correlations between the PROMIS PF-10 and measures of PF and quality of life. RESULTS Data from 316 patients (180 RA/136 axSpA, 91.7%/47.8% female, mean ± sd age 58.6 ± 13.2/54.0 ± 11.3 years) were analysed. The median (IQR) PROMIS PF-10 score was 34.5 (31.4-37.6) in RA and 36.0 (32.8-38.3) in axSpA patients. The PROMIS PF-10 correlated strongly with the HAQ-DI, BASFI, and EQ-5D-5L index score (r > 0.6), moderately with the SF-36 Physical Component Summary score, EQ-VAS, and 6MWT (0.30 ≤ r ≤ 0.60), and weakly with the SF-36 Mental Component Summary score (r < 0.30). Five of six hypotheses (83%) were confirmed in both groups. CONCLUSION The overall strong correlation of the PROMIS PF-10 with measures of PF and moderate to weak correlations with outcomes measuring different constructs were confirmed in subpopulations of RA and axSpA patients with severe functional limitations, supporting its construct validity.