- Last Update On : 2013-03-09
The Sm and nuclear ribonucleoprotein (RNP) antigens are a particulate complex composed of small nuclear RNAs (U-RNAs) and proteins. This complex has also been referred to as extractable nuclear antigens (ENA), since it is soluble in saline. Autoantibodies to these antigens occur in systemic lupus erythematosis and mixed connective tissue disease.
The Sm (Smith) and related nuclear ribonucleoproteins (nRNPs) are targets for autoantibodies in SLE. These antigens are present in subcellular organelles called spliceosomes that are composed of peptide containing small RNAs. Anti-Sm antibodies are only present in 15 to 30% of the patients with SLE, but they are highly specific for SLE. They occur more frequently (60%) in young black females with SLE. They almost never occur in healthy individuals or patients with other diseases. Anti-Sm antibodies should not be confused with anti-smooth muscle antibodies detected in autoimmune liver disease. Anti-RNP antibodies, which are commonly tested for in conjunction with anti-Sm, are present in 30 to 40% of SLE patients. However, anti-RNP antibodies are not specific for SLE and are not useful for establishing the diagnosis of SLE. High titers of Sm and RNP antibodies have been reported in patients with less renal and central nervous system disease, though others have refuted these findings. Sm antibodies may disappear with treatment, while RNP antibodies persist.
Many patients present with clinical signs and symptoms that are compatible with more than one systemic rheumatic disease. One such overlap syndrome is mixed connective tissue disease (MCTD). Patients with MCTD have overlapping features of SLE, scleroderma, and myositis. Arthritis, arthralgia, Raynaud phenomenon, esophageal dysfunction, and myositis are common, but renal involvement is rare. Detection of RNP antibody, in the absence of other antibodies, strongly suggests the diagnosis of MCTD. Two laboratory criteria are necessary to diagnose MCTD: (1) the presence of high titer RNP antibodies and (2) the absence of anti-DNA, anti-Sm, and histone antibodies
With rare exceptions, these tests should not be ordered if the ANA was negative or weakly positive, because less than 5% of patients with ANA titers <1:160 will have positive follow-up tests. Sm titers should not be measured as a marker of disease activity or to establish prognosis.
Results are reported as positive or negative. The reference value is negative.
Specimen requirement is one plain red top tube of blood.