Anti-Carbamylated Protein Antibodies in ACPA-Negative and ACPA-Positive Patients with Rheumatoid Arthritis

The objective of this study was to assess the level of antibodies to carbamylated proteins and analyze the clinical and immunological associations in patients with ACPA-negative and ACPA-positive variants of rheumatoid arthritis.

. The study involved 150 patients with a reliable diagnosis of rheumatoid arthritis and 25 patients as healthy controls. Depending on ACPA values, two groups of patients were recruited: ACPA-positive (n = 75) and ACPA-negative (n = 75). RA activity was assessed by the DAS28 index. Determination of antibodies to carbamylated proteins was performed by enzyme-linked immunosorbent assay (BlueGene Biotech, China). Quantitative determination of ACPA in serum was performed by enzyme immunoassay using a commercial reagent kit (AxisShield, UK; upper limit of normal 5.0 U/mL; Orgentec, Germany; upper limit of normal 20.0 U/mL).

. Median anti-CarP in patients with RA was 126.2 [100.83; 157.41] ng/mL and was statistically significantly higher (p < 0.001) than in healthy controls (88.89 [70.53; 107.75] ng/mL). Among all patients with RA, 50 (33.3%) were anti-Carp-positive (22 (29.3%) in the ACPA(+) group and 28 (37.3%) in the ACPA(–) group), and one (2%) volunteer from healthy controls was anti-CarP(+) (p = 0.002). In ROC analysis performed to assess the diagnostic significance of anti-CarP for RA for all patients with RA, the area under the curve was 0.783 ± 0.047 with 95% CI: 0.691–0.874 (p < 0.001), with a cut-off point of 143 ng/mL, specificity 96%, sensitivity 36.7%.

In the ACPA(+) RA group, the erosion count was statistically significantly higher (p = 0.044) in anti-CarP(+) patients than in anti-CarP(–) patients. A weak direct correlation between anti-CarP and DAS28 was found in the ACPA(–) RA group.

. We studied the predictive value of anti-CarP as an auxiliary biomarker in ACPA(+) and ACPA(–) subtypes of RA. ACPA(+) anti-CarP(+) patients have a more “erosive” subtype of the disease than ACPA(+) anti-CarP(–) patients. In ACPA(–) patients, anti-CarP helps to identify a more erosive subtype of the disease, and among ACPA(–) patients it helps to reduce the proportion of seronegative patients. Further studies are required to determine the optimal standards for the laboratory diagnosis of anti-CarP and to clarify the diagnostic potential of these ABs as part of the differential diagnosis of arthritis in other rheumatic diseases.