Cohort profile: SCREEN-RA: design, methods and perspectives of a Swiss cohort study of first-degree relatives of patients with rheumatoid arthritis oleh Kim Lauper, Denis Mongin, Delphine S Courvoisier, Celine Lamacchia, Diego Kyburz, Andrea Rubbert-Roth, Benoît Thomas P Gilbert, Eric Trunk, Olivia Studer, Burkhard Moeller