Background: Anti-Smith (Sm) antibody is a highly specific antibody for systemic lupus erythematosus (SLE). Despite the remarkable specificity of anti-Sm antibodies for SLE, the association between anti-Sm antibody level and the clinical manifestation of SLE is still unclear.
Objectives: We aimed to evaluate the association between anti-Sm antibodies and disease activity in patients with new-onset SLE.
Methods: We included patients who were repeatedly tested for anti-Sm antibodies at SLE diagnosis and within 12 months after SLE diagnosis. The clinical and laboratory profiles, and SLE disease activity index (SLEDAI) were collected at the time of the anti-Sm antibody test. SLEDAI and laboratory variables associated with disease activity were compared at baseline between patients with and without anti-Sm antibodies. The longitudinal association between disease activity and anti-Sm antibodies was also evaluated in total patients and in those with anti-Sm antibodies.
Results: Of 92 patients who were tested for anti-Sm antibodies at the time of SLE diagnosis, 67 and another 67 patients were followed up for the presence of anti-Sm antibodies at 6 and 12 months, respectively. Although the baseline SLEDAI was comparable in SLE patients with and without anti-Sm antibodies, the serum anti-Sm antibody level at diagnosis was significantly correlated with SLEDAI (P = 0.003). Patients with anti-Sm antibodies at 12 months had higher SLEDAI and anti-dsDNA levels than those without anti-Sm antibodies (P = 0.002, respectively). The changes in anti-Sm antibody levels over 12 months were also correlated with the alterations in SLEDAI (P = 0.029).
Conclusion: This study suggests that anti-Sm antibody level is associated with disease activity in patients with new-onset SLE, and that monitoring of anti-Sm antibody levels could help assess the disease activity.
References  Migliorini, P., et al., Anti-Sm and anti-RNP antibodies. Autoimmunity, 2005. 38(1): p. 47-54.
 Clinical associations of anti-Smith antibodies in PROFILE: a multi-ethnic lupus cohort. Clin Rheumatol, 2015. 34(7): p. 1217-23.
 What is the clinical significance of anti-Sm antibodies in systemic lupus erythematosus? A comparison with anti-dsDNA antibodies and C3. Clin Exp Rheumatol, 2017. 35(4): p. 598-606.
Disclosure of Interests: None declared
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