Antibodies to ribonucleoprotein, RNP, are found often in very high levels in patients with a variety of systemic rheumatic diseases, including SLE, progressive systemic sclerosis and Mixed Connective Tissue Disease, a distinct disease entity with overlapping clinical features of SLE, scleroderma, polymyositis and rheumatoid arthritis. These autoantibodies usually give a strong speckled pattern of nuclear staining in immunofluorescence. The structure of the antigens recognised by anti-RNP antibodies is complex. The commonest determinants and ones which are most widely detected are those called RNP or U1RNP. Anti-RNP antibodies can be detected alone (usually in MCTD) or together with anti-Sm (lupus). Anti-Sm is seldom found alone.
Autoantibody found in SLE (often with Sm) or alone in Mixed connective tissue disease.
Antibodies to RNP are found in very high levels in 95-100% of patients with mixed connective tissue disease. Indeed, if a high titre of RNP is found and other ANA specificities are absent this is synonymous with MCTD. There remains debate as to whether MCTD represents a truly distinct disease or merely an overlap syndrome. However the diagnostic serology is clear even though the full range of clinical involvement is not apparent at that time.In other diseases, anti-RNP antibodies are frequently associated with antibodies to Sm proteins. Antibody to RNP and Sm antigen are found in around 20-30% of patients with SLE. 20-25% of patients with progressive systemic sclerosis have anti-RNP. They are also found in other connective tissue diseases including a small <1% minority of patients with Sjögrens syndrome or rheumatoid arthritis. Anti-Sm is seldom found alone.
Serum Separator Tube (SST)
ImmunoCAP 250 Human recombinant RNP70, A and C proteins) IgG antibodies.
Test is only carried out when ENA screen is positive.
Results reported as negative, equivocal or positive.
Immunology The James Cook University Hospital