Do all anti-citrullinated protein/peptide antibody tests measure the same? Evaluation of discrepancy between anti-citrullinated protein/peptide antibody tests in patients with and without rheumatoid arthritis.

BACKGROUND

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

AIMS

To evaluate discrepancy between four ACPA tests.

PATIENTS AND METHODS

Population 1 consisted of patients with a new diagnostic problem, including 86 patients with rheumatoid arthritis (RA) and 450 patients without RA. Population 2 consisted of 155 patients with RA who had long-standing disease. Population 3 consisted of 188 patients with psoriatic arthritis and in population 4 there were 192 patients with systemic lupus erythematosus. Populations 1 and 2 were tested with the anti-human fibrinogen antibody (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-analysed in a different run. Populations 3 and 4 were analysed in the CCP2-euro and AhFibA test.

RESULTS

In population 1, ACPA positivity was found in 17 of 450 (3.8%) patients without RA; 14 (82%) of these 17 samples were discrepant. In contrast, 61 of 86 (70.9%) patients with RA were ACPA positive of whom 18 of 61 (29.5%) 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 patients with systemic lupus erythematosus who were ACPA positive than in those with psoriatic arthritis who were ACPA positive.

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.