The DIASTAT®anti-Sm/RNP test is a quantitative/qualitative enzyme-linked immunosorbent assay (ELISA) for the detection of autoantibodies specific for Sm and RNP antigens in human serum or EDTA, lithium heparin, citrated plasma. In conjunction with the anti-Sm test, it is intended to aid in the diagnosis of systemic lupus erythematosus and mixed connective tissue disease and is not definitive in isolation. Autoantibody levels represent one parameter in a multicriterion diagnostic process.
Systemic rheumatic diseases are autoimmune disorders such as systemic lupus erythematosus (SLE), polymyositis, Sjögren's syndrome, scleroderma and mixed connective tissue disease (MCTD). A general feature of systemic rheumatic diseases is the presence of circulating antibodies to a variety of cellular antigens. The detection and serological characterisation of specific autoantibodies plays an important role in the differential diagnosis of these diseases.
The U1-RNP antigen is a ribonucleoprotein; antibodies against it are found in several connective tissue diseases, principally SLE (30-40%) and MCTD (>95%). MCTD presents a combination of clinical features, which may also be seen in SLE, polymyositis and scleroderma. Although antibodies to dsDNA, Sm and Ro are occasionally found in MCTD, high titres of anti-RNP antibodies are usually considered diagnostic of the disease. In addition, a number of reports have noted the variation of anti-RNP titre with the course of SLE. This confirms the need for an accurate and easy-to-use test.
The wells of the microtitre strips are coated with affinity-purified Sm/RNP antigen. During the first incubation, specific autoantibodies in diluted serum or plasma bind to the antigen-coated surface. The wells are then washed to remove unbound components. In the second incubation, the Conjugate, enzyme-labelled antibodies to human IgG, binds any surface-bound autoantibodies. After further washing, specific autoantibodies are traced by incubation with the Substrate. Addition of Stop Solution terminates the reaction, resulting in a coloured end-product. The amount of Conjugate bound is measured in absorbance units. In the qualitative protocol, the amount of Conjugate bound by the sample is compared with that bound by the Reference Control. In the quantitative protocol, the concentration of anti‑Sm/RNP autoantibody can be estimated by interpolation from a dose-response curve based on Standards.