title: Different properties of ACPA and IgM-RF derived from a large dataset: further evidence of two distinct autoantibody systems.
abstract: BACKGROUND
  The aim of this study was to examine seroconversion and the relationship with age and inflammation of autoantibodies in a large group of patients attending an outpatient rheumatology clinic.
  
  
  METHODS
  Levels of antibodies to citrullinated proteins/peptides (ACPAs) and IgM rheumatoid factor (IgM-RF) were determined in 22,427 samples collected from 18,658 patients. The diagnosis was derived from a diagnosis registration system. The degree of seroconversion in repeated samples and the correlation of levels with age and inflammatory markers were determined for ACPA and IgM-RF in rheumatoid arthritis (RA) and non-RA patients.
  
  
  RESULTS
  Seventy-one percent of RA patients (n = 1,524) were ACPA-positive and 53% were IgM-RF-positive; in non-RA patients (n = 2,245), the corresponding values were 2% and 4%, respectively. In patients with at least two samples (n = 3,769), ACPA status was more stable than IgM-RF status in RA patients. ACPA- or IgM-RF-negative non-RA patients seldom became positive. ACPA positivity was unrelated to age in both RA and non-RA patients. IgM-RF positivity was unrelated to age in RA patients; however, it increased with age in non-RA patients. The correlation between autoantibody levels and inflammatory markers was low in general and was somewhat higher for IgM-RF than for ACPA.
  
  
  CONCLUSIONS
  ACPA status is more stable in time and with increasing age than IgM-RF status, further establishing its role as a disease-specific marker. ACPA and IgM-RF levels are only moderately correlated with markers of inflammation.
authors.affiliation: Jan van Breemen Institute, 1056 AB Amsterdam, The Netherlands. j.ursum@janvanbreemen.nl
authors.forename: Jennie
authors.initials: J
authors.lastname: Ursum
authors.forename: Wouter H
authors.initials: WH
authors.lastname: Bos
authors.forename: Rob J
authors.initials: RJ
authors.lastname: van de Stadt
authors.forename: Ben A C
authors.initials: BA
authors.lastname: Dijkmans
authors.forename: Dirkjan
authors.initials: D
authors.lastname: van Schaardenburg
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001172:Arthritis, Rheumatoid / Q000097:blood / Q000175:diagnosis / Q000276:immunology; D001323:Autoantibodies / Q000096:biosynthesis / Q000097:blood*; D015415:Biomarkers / Q000097:blood; D002956:Citrulline / Q000097:blood* / Q000276:immunology; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007075:Immunoglobulin M / Q000096:biosynthesis / Q000097:blood*; D008297:Male; D008875:Middle Aged; D012217:Rheumatoid Factor / Q000096:biosynthesis / Q000097:blood*
chemical_list: D001323:Autoantibodies; D015415:Biomarkers; D007075:Immunoglobulin M; D002956:Citrulline; D012217:Rheumatoid Factor