Background Beside Rheumatoid Factor (RF) and anti-citrullinated peptide antibodies (ACPA), crucial biomarkers in the diagnosis of Rheumatoid Arthritis (RA), antibodies against carbamylated proteins (anti-CarP) have been recently identified in the sera of RA patients [1, 2]. Despite the interesting data presented up to now regarding anti-CarP in the sera of RA patients, the specificity and sensitivity of these new autoantibodies have not been addressed yet [3].

Objectives To evaluate prevalence, sensitivity and specificity of anti-CarP compared to ACPA and RF, in a large cohort of RA patients, normal healthy subjects (NHS) and other disease controls.

Methods A total of 607 frozen stored sera from 309 patients with established RA diagnosed according to the 2010 ACR/EULAR criteria, 98 NHS and 200 patients with other rheumatic conditions were evaluated. Sera were collected from consecutive unselected patients attending outpatient clinics of our department for clinical evaluation. The disease control group included patients with Systemic Lupus Erythematosus (SLE) (n=83), Sjögren's syndrome (SS) (n=45), systemic sclerosis (SCL) (n=51), and osteoporosis (OP) (n=21). Anti-CarP antibodies were detected by a modified solid phase “home-made” ELISA as described by Shi et al. using carbamylated foetal calf serum (FCS) as antigen [4]. ACPAs were detected using a second-generation ELISA (anti-CCP) kit (Delta Biologicals) while IgM RF was determined as part of routine analysis by immunonephelometry (Behering, Marburg, Germany).

Results Anti-CarP antibodies were detected in 117 (34.4%) of RA patients, ACPA in 190 (61.4%) while RF in 202 (65.3%) of them. Two (2.04%) of the NHS were positive for anti-CarP, one (1.02%) for ACPA and three for RF (3.06%). Among disease controls, anti-CarP antibodies were detected in 14 (16.8%) of the SLE patients, 14 (31.1%) of SS patients, 3 (5.8%) of SCL patients and 2 (9.5%) of patients with OP. ACPAs resulted positive in 15 (18.07%) of the SLE patients, 11 (24.4%) of SS patients, 1 (1.9%) of SCL patients and 2 (9.5%) of patients with OP. On the other hand RF was detected in 19 (22.8%) of the SLE patients, 23 (51.1%) of SS patients, 18 (35.2%) of SCL patients and 4 (19.04%) of patients with OP. All data are summarised in Figure 1. To note, the sensitivity of anti-CarP, ACPAs and RF was 46.8%, 61.8% and 64.4% respectively while specificity was recorded as 91.95%, 89.93% and 76.51% respectively.

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Conclusions Anti-CarP antibodies determined by the reactivity to carbamylated FCS, are predominantly present in RA but can also be detected in other rheumatic conditions. The prognostic relevance of anti-CarP antibodies in these latter patients should be further determined. Among the three autoantibodies studied, anti-CarP antibodies showed a slightly higher specificity but a relatively low sensitivity.

1. Willemze A., Neth J Med 2012.

2. Shi J et al., Ann Rheum Dis 2014.

3. Shi J et al., Autoimmun Rev 2014.

4. Shi J et al., Proc Natl Acad Sci USA 2011.

Disclosure of Interest None declared