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
- rheumatoid arthritis