Anti-carbamylated Protein Antibodies Positivity in Rheumatoid Arthritis and Its Association With Rheumatoid Factor and Anti-cyclic Citrullinated Protein Antibodies

Abstract

Background Rheumatoid arthritis (RA) is a widespread autoimmune disease affecting millions of people worldwide. The current markers include anti-cyclic citrullinated peptide (anti-CCP) antibodies and rheumatoid factor (RF), which are nonspecific and elevated in various conditions and do not have a prognostic value. They are also elevated in the later stages of the disease. Anti-carbamylated protein (anti-CarP) antibodies have been reported to be associated with joint damage in RA. Therefore, this study aimed to evaluate the sensitivity and specificity of anti-CarP antibodies in individuals with RA and their relationship with inflammatory markers such as C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). Methods This was a cross-sectional case-control study conducted from April 2020 to March 2021 at the Saveetha Medical College, Chennai, India. The age makeup of the three groups was evaluated: Group 1 comprised anti-CCP and RF-positive patients; Group 2 comprised anti-CCP and RF-negative patients; and Group 3 was the control group, which comprised healthy volunteers. Patient samples, including blood and serum, have been utilized to conduct various assessments aimed at evaluating biomarkers such as CRP, ESR, RF, and autoantibodies like anti-CCP and anti-CarP. Results This study examined the role of various autoantibodies and biomarkers in RA across three distinct groups. Group 1 predominantly consisted of middle-aged individuals, and women constituted the majority in both Group 1 and Group 2, consistent with higher RA prevalence among females. In Group 1, 65.7% of patients tested positive for anti-CarP, while in Group 2, 48.6% tested positive even when RF and anti-CCP antibodies were absent. This suggests a potential diagnostic role for anti-CarP antibodies in identifying RA patients early. CRP and ESR levels were significantly elevated in RA patients (Groups 1 and 2) compared to healthy controls (Group 3), indicating higher inflammatory activity in affected individuals. We also observed that anti-CarP antibodies had a specificity of 69.1% and a sensitivity of 78.2%. Positive correlations between the diagnosis of RA and anti-CarP antibody positivity were observed across the groups and correlated well with the inflammatory markers and signs such as joint damage. The data were found to be statistically significant. Conclusions Our study showed a significant correlation between joint damage and CRP levels and a positive correlation between anti-CarP antibodies and ESR and CRP values. These findings suggest that anti-CarP antibodies can offer certain advantages over RF and anti-CCP antibodies in RA diagnosis due to their early detection potential, higher specificity, complementary diagnostic role, and predictive value for disease severity.

Keywords: anti-carp antibodies; anti-ccp antibodies; autoimmune disease; ra factor; rheumatoid arthriitis.

Figure 1. Distribution of age among rheumatoid arthritis patients and healthy controls.

This figure represents the data in the form of percentages (N1=35, N2=35, N3=20). A p-value less than 0.05 is considered statistically significant; here, the p-value was found to be 0.374 by the Fisher exact test.

Figure 2. Sex distribution among patients with rheumatoid arthritis and healthy controls using the chi-square test.

This figure represents the data in the form of percentages (N1=35, N2=35, N3=20). A p-value less than 0.05 is considered statistically significant; here, the p-value was found to be 0.960 by Pearson's chi-square test.

Figure 3. Association of erythrocyte sedimentation rate (ESR) between rheumatoid arthritis patients and healthy controls using the chi-square test.

This figure represents the data in the form of percentages (N1=35, N2=35, N3=20). A p-value less than 0.05 is considered statistically significant; here, the p-value was found to be 0.000, which makes the data statistically significant. ESR: erythrocyte sedimentation rate.

Figure 4. Association of joint damage between rheumatoid arthritis patients and healthy controls.

This figure represents the data in the form of percentages (N1=35, N2=35, N3=20). A p-value less than 0.05 is considered statistically significant; here, the p-value was found to be 0.000, which makes the data statistically significant.

Figure 5. Distribution of anti-carbamylated protein antibodies (biomarkers) in joint damage in rheumatoid arthritis and healthy control groups.

This figure represents the data in the form of percentages (N1p=23, N1n=12, N2p=17, N2n=18, N3p=2, N3n=18 where p indicates positivity for the corresponding antibody and n indicates negativity for the same). A p-value less than 0.05 is considered statistically significant; here, the p-value was found to be 0.000, which makes the data statistically significant. Anti-CarP: anti-carbamylated protein.

Figure 6. ROC curve for the diagnostic value of anti-CarP antibody for RA.

The anti-CarP antibody was tested using ELISA. The cutoff value for positivity was defined using the ROC curve as 4.2 U/mL, with a specificity of 69.1%, a sensitivity of 78.2%, an AUC of 0.782, a 95% CI, and p<0.0001. RA: rheumatoid arthritis; anti-CarP: anti-carbamylated protein; ELISA enzyme-linked immunosorbent assay; ROC: receiver operating characteristic; AUC: area under the curve; CI: confidence interval.