ANTI SMITH / U-nRNP – IgG
High titres of antibodies against U-nRNP are characteristic of mixed collagenosis (MCTD,Sharp syndrome ).The prevalence is 95 – 100%. The antibody titre correlates with the level of activity of the disease.
These antibodies also occur in cases of SLE , butthen almost always in combination with anti smith.
Antibodies against Smith possess a high specificity for SLE . Together with anti-dsDNA,they can be considered to be pathognomonic for this condition,although they occur only in 20-40% of patients.