Advancing Stiffness Measurement in Rheumatic Disease: Report from the Stiffness Special Interest Group at OMERACT 2018.

OBJECTIVE:To improve measurement of stiffness in rheumatic disease. METHODS:Data presented included (1) 2 qualitative projects, (2) the rheumatoid arthritis (RA) stiffness patient-reported outcome measure (RAST), and (3) 3 items assessing stiffness severity, duration, and interference. RESULTS:Stiffness is multidimensional and includes aspects of stiffness experience such as duration, severity, and effect. Stiffness items showed construct validity in RA. Further efforts are required to develop an instrument that will be taken through the Outcome Measures in Rheumatology (OMERACT) Filter 2.1 for instrument selection. CONCLUSION:The research agenda for the group includes domain content voting for individual diseases, and development of stiffness item banks and disease-specific short forms.