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Do all anti-citrullinated protein/peptide antibody (ACPA) tests measure the same? Evaluation of discrepancy between ACPA tests in RA and non-RA patients.
  1. Bert Vander Cruyssen (bert.vandercruyssen{at}
  1. UZ Gent, Belgium
    1. Leonor Nogueira (nogueira.l{at}
    1. UMR 5165 "Laboratory of Epidermis Differentiation and Rheumatoid Autoimmunity", France
      1. Jens Van Praet (jens.vanpraet{at}
      1. Ghent university hospital, Belgium
        1. Dieter Deforce (dieter.deforce{at}
        1. Laboratory of Pharmaceutical Biotechnology, Ghent University, Belgium
          1. Dirk Elewaut (dirk.elewaut{at}
          1. University Hospital Gent, Belgium
            1. Guy Serre (serre.sec{at}
            1. UMR 5165 "Laboratory of Epidermis Differentiation and Rheumatoid Autoimmunity", France
              1. Filip de Keyser (filip.dekeyser{at}
              1. University Hospital Ghent, Belgium


                Background: Different methods exist to demonstrate anti-citrullinated protein/peptide antibodies (ACPA).

                Aims: To evaluate discrepancy between 4 ACPA tests.

                Patients and Methods: Population 1 consisted of patients with a new diagnostic problem including 86 RA and 450 non-RA patients. Population 2 consisted of 155 RA patients with longstanding disease. Population 3 consisted of 188 psoriatic arthritis (PsA) patients and population 4 consisted of 192 lupus (SLE) patients. Populations 1 and 2 were tested with the anti-human fibrinogen antibodies (AhfibA) test, anti-CCP2 from Eurodiagnostica (CCP2-euro), anti-CCP2 from Pharmacia (CCP2-phar) and anti-CCP3 test by Inova (CCP3). Samples were annotated as discrepant if positive in one and negative in at least one other test. Each discrepant sample was re-analyzed in a different run. Populations 3 and 4 were analyzed in the CCP2-euro and AhFibA test.

                Results: In population 1, ACPA positivity was found in 17 of 450 (3.8%) non-RA patients. 14 (82%) of those 17 samples were discrepant. In contrast, 61 (70.9%) of 86 RA patients were ACPA positive of whom 18 (29.5%) out of 61 were discrepant (70.9% vs. 29.5%, p<0.001). The discrepancies between tests could be partly attributed to borderline results, inter-assay discrepancy and inter-test variability. They were more prevalent in ACPA positive SLE patients than in ACPA positive PsA patients.

                Conclusions: Discrepancy between different ACPA tests was observed attributable to the occurrence of borderline results, inter-assay variability and mainly to inter-test variability. The lowest inter-test discrepancy is observed between tests that use the same substrate.

                • anti-citrullinated protein/peptide antibodies
                • discrepancy
                • rheumatoid arthritis

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