Antibodies to citrullinated protein/peptide (ACPA) are established biomarkers for diagnosis and classification of rheumatoid arthritis (RA).1 Different generations of ACPA immunoassays are commercially available, with differences in antigen used (eg, citrullinated recombinant rat filaggrin (CCP1), synthetic cyclic citrullinated peptides (CCP2), multiple citrullinated epitopes (CCP3), and mutated and citrullinated vimentin (MCV)).2 There is poor agreement among commercially available ACPA assays.3 To facilitate comparison between ACPA assays, adoption of an international standard is desirable.

The International Union for Immunological Societies (IUIS) and Centers for Disease Control and Prevention (CDC) prepared an ACPA reference preparation (www.autoab.org) in 2008, which has not generally been adopted as a reference standard for establishing calibration curves. In 2019, the National Institute for Biological Standards and Control (NIBSC), known as the Medicines and Healthcare Regulatory Agency, developed a standard preparation NIBSC-18/204 for ACPA derived from a serum pool of five patients with RA. Based on the results of a large international study, the WHO acknowledged NIBSC-18/204 as the first WHO International Standard for ACPA.4

In 2022, we evaluated the NIBSC-18/204 standard using nine commercially available CCP2 assays.5 However, in this study, a CCP3 assay was lacking. In the current …