title: Should anti-citrullinated protein antibody and rheumatoid factor status be reassessed during the first year of followup in recent-onset arthritis? A longitudinal study.
abstract: OBJECTIVE
  Presence and levels of antibodies contribute to the classification of rheumatoid arthritis. We investigated the longitudinal course of anti-citrullinated protein antibodies (ACPA) and immunoglobin M (IgM) rheumatoid factor (RF) during the first year after arthritis onset in patients with very short disease duration.
  
  
  METHODS
  Patients (aged 18-75 years) with ≥ 1 swollen joint of ≤ 16 weeks' duration had assessments of ACPA (2nd generation anti-cyclic citrullinated peptide antibodies, anti-CCP2) and IgM RF at inclusion and after 3, 6, and 12 months. Frequencies of seroconversions (negative to positive and vice versa) and changes in antibody levels during followup were determined.
  
  
  RESULTS
  A total of 281 early arthritis patients (median duration of joint swelling 32 days, 14.2% ACPA positives, 12.8% IgM RF positives) with 978 longitudinally collected serum samples were included. Only 5 patients (1.8%) negative for both antibodies at baseline turned antibody-positive during followup, while 9 antibody-positive patients (3.2%) turned antibody-negative. ACPA was more stable than RF regarding both status and levels.
  
  
  CONCLUSIONS
  Antibody status (ACPA/RF) is a stable phenotype in very early arthritis, as seroconversion was only found in 5% of patients. Repeated measurement of ACPA or RF during the first year after onset of arthritis does not offer major additional information.
authors.affiliation: Department of Rheumatology, Diakonhjemmet Hospital, Oslo, Norway. maria_mjaavatten@hotmail.com
authors.forename: Maria D
authors.initials: MD
authors.lastname: Mjaavatten
authors.forename: Désirée M
authors.initials: DM
authors.lastname: van der Heijde
authors.forename: Till
authors.initials: T
authors.lastname: Uhlig
authors.forename: Anne J
authors.initials: AJ
authors.lastname: Haugen
authors.forename: Halvor
authors.initials: H
authors.lastname: Nygaard
authors.forename: Olav
authors.initials: O
authors.lastname: Bjørneboe
authors.forename: Tore K
authors.initials: TK
authors.lastname: Kvien
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000888:Antibodies, Anti-Idiotypic / Q000097:blood*; D001172:Arthritis, Rheumatoid / Q000097:blood* / Q000145:classification / Q000175:diagnosis*; D015331:Cohort Studies; D004185:Disability Evaluation; D018450:Disease Progression; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007075:Immunoglobulin M / Q000097:blood; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010456:Peptides, Cyclic / Q000276:immunology*; D010641:Phenotype; D012217:Rheumatoid Factor / Q000097:blood*; D012720:Severity of Illness Index; D055815:Young Adult
chemical_list: D000888:Antibodies, Anti-Idiotypic; D007075:Immunoglobulin M; D010456:Peptides, Cyclic; C487763:cyclic citrullinated peptide; D012217:Rheumatoid Factor