The DIASTAT® anti-Sm test is a quantitative/qualitative enzyme-linked immunosorbent assay (ELISA) for the detection of autoantibodies specific for Sm antigen in human serum or EDTA, lithium heparin, citrated plasma. It is intended to aid in the diagnosis of systemic lupus erythematosus 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. 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.
Sm is a non-histone glycoprotein; antibodies against this extractable nuclear antigen are specific serological markers of SLE. Antibody levels may fluctuate with time and disease course, confirming the need for quantitative analysis.About 10-30% of patients with SLE in Western Europe are positive for Sm antibodies. In SLE-patients with afro-American and Asian origin Sm-antibodies are more frequently detected than in SLE-patients with Caucasian origin.
The wells of the microtitre strips are coated with affinity-purified Sm 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 autoantibody can be estimated by interpolation from a dose-response curve based on Standards.