Sensitivity and specificity of anti-Sa autoantibodies for rheumatoid arthritis.

Objective: To assess the sensitivity and specificity of the recently described anti-Sa autoantibodies in order to determine their potential usefulness for the diagnosis of rheumatoid arthritis (RA).

Methods: Sera from 67 patients with RA (including 31 patients with early RA of <3 months duration), from 180 patients with other rheumatic diseases and from 30 healthy control subjects were investigated by immunoblotting employing partially purified Sa antigen. Additionally, all sera were tested for rheumatoid factor (RF), anti-A2/RA33, antinuclear antibodies (ANA) and ANA subsets.

Results: Twenty-one (31%) of the 67 patients with RA, but only four of the 180 control patients, were anti-Sa positive; therefore, anti-Sa was approximately 98% specific for RA. Anti-Sa was not associated with either RF or with anti-A2/RA33. The latter antibody was present in 21 RA sera, only eight of which also contained anti-Sa. Thus, 34 RA sera (51%) showed at least one of these two autoantibodies and, importantly, 18 of these sera were RF negative. Furthermore, of the 31 patients with early RA, 12 (40%) were anti-Sa and/or anti-A2/RA33 positive.

Conclusion: Although the numbers studied remain small, taken together, these data suggest that anti-Sa may represent a promising novel serological marker with high specificity for RA.