Acknowledgements
The cohort study was partially financed by AbbVie. Measurement of drug levels was sponsored by Pfizer. We are grateful to the clinical Research Bureau of the Jan van Breemen Research Institute, location Reade, which receives support from the Dutch Arthritis Foundation, for help in the conduct of the study. We also thank the research nurses and medical doctors for performing clinical assessments, and the technicians of Sanquin Diagnostics for performing the assays.