Fig 1. Flow diagram of the selection of patients in this study.

Patients were recruited among all patients diagnosed with rheumatoid arthritis and treated with a TNFi, with serum samples available at the research biobank at the Parker Institute, Copenhagen University Hospital at Frederiksberg and Bispebjerg, Denmark. ^A Only patients treated with adalimumab or infliximab were included, as we did not have access to a reliable detection method regarding antibodies towards etanercept. ^B Vials were adequate only if they had not been previously thawed. At baseline, one vial of ≥ 3μl was required, and at 6 months follow-up 2 vials of ≥ 3 μl were required. ^C Missing samples due to inclusion in project not requiring 6 months sampling, termination of current treatment, or lost to follow-up.