Objectives: To evaluate in various groups of patients with chronic joint disease the sensitivity and specificity of anti-Sa antibody, recently described in sera from adults with rheumatoid arthritis (RA); and to determine the prognostic significance of anti-Sa in initial sera from patients with long standing RA with or without severe joint destruction.
Methods: Serum samples from 489 patients were included. Of these, 154 were collected from patients with RA attending 2 rheumatology units. Controls were 335 patients with a variety of inflammatory joint diseases other than RA. IgG anti-Sa was detected using an immunoblotting method with purified Sa antigen from human placenta extracts. All patients were tested for the following antibodies: rheumatoid factor (RF), anti-keratin antibody (AKA), antiperinuclear factor (APF), and anti-RA 33. HLA class II DRB alleles were also determined.
Results: Anti-Sa was detected in 39.8% of RA sera overall, 46.7% of sera from the long standing RA group, and 23.5% of sera from the recent onset RA group (p<0.01). In patients with long standing RA, statistically significant associations were found between the presence of anti-Sa and the following variables: RF (p<0.0001), AKA (p<0.0001), APF (p<0.00001), and HLA DRB1*04 or 01 (p<0.01). In contrast, no association was found with anti-RA33. Anti-Sa was positive in 11 adult controls (7.8%) and in 26 pediatric patients with juvenile chronic arthritis (22%). The specificity of anti-Sa for RA was 92.1% in adults with well characterized rheumatic diseases and 85.9% in adults and children together. Among patients with long standing RA, those with destructive disease were more likely to test positive for anti-Sa (66.6%) than those with nondestructive disease (22.2%) (p<0.0001). Comparisons with other serologic markers for RA demonstrated that anti-Sa was sensitive (68.4%) and was also the test with the highest specificity (79%), positive predictive value (75%), and negative predictive value (71%) for discriminating between patients who do and those that do not develop late severe radiographic damage.
Conclusion: Immunoblot-detected IgG anti-Sa is a sensitive serologic marker for RA patients with severe radiographic damage.