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Only rheumatoid factor‐positive subset of anti‐citrullinated peptide/protein antibody‐negative rheumatoid arthritis may seroconvert to anti‐citrullinated peptide/protein antibody‐positive

Anti‐citrullinated peptide/protein antibody (ACPA) has been reported to occur in about 60% of patients with early rheumatoid arthritis (RA), and about 80% in patients with established RA. While ACPA seroconversion is possible, previous reports have shown that it rarely occurs. We retrospectively determined the proportion of patients who underwent ACPA seroconversion and described the clinical characteristics of these cases.