Footnotes

• Handling editor Josef S Smolen

• Contributors LVH, XB, JD and LEB-J contributed to the study conception and design. Patient samples were selected by JD, LEB-J and CB. Reference material was provided by MHRA (DV and LS). Material preparation and data collection were performed by JD, LEB-J and CB; data analysis by LVH and XB. Manuscript was prepared by LVH, JD and XB and commented by all authors. The final manuscript was read and approved by all authors. XB acts as the guarantor.

• Funding Thermo Fisher Scientific donated the ACPA assays and provided technical training and support free of charge.

• Competing interests LVH, GS and DS have received lecture fees from Thermo Fisher Scientific and have been a consultant for Thermo Fisher Scientific; XB has received lecture fees from Werfen and from Thermo Fisher Scientific; JD has received consultancy and/or speakers fees from Werfen/Inova, ThermoFisher Scientific and Euroimmun.

• Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

• Provenance and peer review Not commissioned; externally peer reviewed.