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.
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.
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.
Immunoblot-detected IgG anti-Sa is a sensitive serologic marker for RA patients with severe radiographic damage.