Alternative name:
Description: Small nuclear RNP (snRNP) particles often termed spliceosomes are composed of small RNA molecules, together with associated proteins including snRNPs which contain an Sm core together with proteins which are specific to that type of particle. The well-characterised RNP autoantigens are located primarily in U1 snRNP complexes where they are involved in pre-mRNA splicing. Sm epitopes are expressed on the snRNP associated proteins which are termed B B1, D, E/F and G. The epitopes of principal reactivity appear to be within D and B/B1.
Indication: Found in SLE (almost always with anti RNP).
Interpretation: Anti-Sm antibodies are almost always associated with antibodies to RNP proteins. Antibody to RNP and Sm antigen are found in around 20-30% of patients with SLE. Most patients with MCTD and around 20-25% of patients with progressive systemic sclerosis have anti-RNP but no Anti-Sm.Sm antibody was considered a highly specific marker for patients with SLE because it has not been detected in normal individuals or in patients with other systemic rheumatic diseases. However, the high incidence (approximately 25 to 30% of patients with SLE) originally reported in the literature was in a population of black Americans with SLE who appear to have a much higher incidence. Some reports suggest anti-Sm to be associated with a milder form of the disease having less renal or CNS involvement whilst Raynauds phenomenon and sclerodactyly were more frequent.
Assay details: Fluorescence enzyme linked immunoassay (Phadia Immunocap 250). Native bovine Sm proteins IgG antibodies.
Restrictions: Test is only carried out when ENA screen is positive.
Reference Range: Results reported as negative, equivocal or positive.
Assay Range Notes:
Turnaround Time: 5 – 7 days
Analysing Laboratory: Immunology The James Cook University Hospital