This study aimed to evaluate the clinical features in patients with suspected connective tissue disease who tested positive for anti-Sm/RNP common motif antibody with or without associated anti-RNP antibody.

The titers of anti-Sm/RNP and anti-RNP antibodies were measured by the multiplex solid-phase bioassays (Bio-Rad). Clinical manifestations were compared among the three subgroups (RNP only, Sm/RNP only, and double positive for RNP and Sm/RNP). Patients were further evaluated for the diagnosis of mixed connective tissue disease (MCTD) and/or systemic lupus erythematosus (SLE) using accepted classification criteria.

A total of 133 patients were included in this study. The rates of inflammatory arthritis and Raynaud phenomenon were significantly higher in patients testing positive for both anti-RNP and anti-Sm/RNP antibodies compared to anti-RNP only or anti-Sm/RNP only (69.1% vs 28.8% vs 25.0%, P < 0.0001 for arthritis and 59.5% vs 23.3% vs 37.5%, P = 0.0005 for Raynaud phenomenon). Area under the curve (AUC) values were 0.68 (95% confidence interval [CI] 0.59-0.77, P < 0.0001) for anti-Sm/RNP titers and 0.65 (95% CI 0.55-0.74, P = 0.0039) for anti-RNP titers with inflammatory arthritis. AUC values were 0.67 (95% CI 0.58-0.77, P = 0.0002) for anti-Sm/RNP titers and 0.59 (95% CI 0.49-0.69, P = 0.0352) for anti-RNP titers with Raynaud phenomenon. The odds ratios for the diagnosis of MCTD and SLE were significantly higher in patients with double positivity compared to those testing solely positive for anti-RNP antibody.

Anti-Sm/RNP common motif autoreactivity when combined with anti-RNP antibody positivity identifies those patients who are closely related with certain clinical manifestations and who are associated with well-defined connective tissue disease such as MCTD or SLE.