Description:

Systemic autoimmune disease is characterized by the presence of circulating auto antibodies directed to a wide variety of cellular antigens. Systemic lupus erythematosis (SLE) commonly referred to as Lupus is the best known of these diseases. Other possible connective tissue diseases include mixed connective tissue disease (MCTD), Sjogren syndrome, sclerodema, and polymyositis/dermatomyositis. The majority can be diagnosed by clinical presentation and their antibody profiles to the various antigens involved, which include dsDNA, Sm, RNP, Ro, La, Scl-70, Jo1 and Histones. Therefore, immunoassays for auto antibodies are useful for diagnostic and prognostic evaluations of autoimmune disease. The Sm or Smith antigen is composed of nuclear RNA and several polypeptides. Antibodies to Sm are present in approximately 30% of patients with SLE. Sm is a very specific marker for SLE. Sm antibodies are very rare in other autoimmune diseases and normals. Classically, antibodies to autoimmune antigens are detected by double immunodiffusion. However, the test is lengthy and suffers from weak sensitivity. Enzyme-Linked Immunosorbent Assays (ELISAs) combine greater sensitivity with ease of use. Many ELISAs have been developed and validated for detecting auto antibodies to various antigens.

The Diagnostic Automation Smith (Sm) test is an Enzyme-Linked Immunosorbent Assay ELISA test to detect IgG, IgA, and IgM antibodies to Smith (Sm) antigens. Purified Smith (Sm) antigens are attached to a solid phase microassay well. Enzyme-Linked Immunosorbent Assays (ELISA) relies on the ability of biological materials (i.e., antigens) to adsorb to plastic surfaces such as polystyrene (solid phase). When antigens bound to the solid phase are brought into contact with a patients serum, antigen specific antibody, if present, will bind to the antigen on the solid phase forming antigen antibody complexes. Excess antibody is removed by washing. This is followed by the addition of goat anti-human IgG, IgA, IgM conjugated with horseradish peroxidase which then binds to the antibody-antigen complexes. The excess conjugate is removed by washing, followed by the addition of Chromogen/Substrate, tetramethylbenzidine (TMB). If specific antibody to the antigen is present in the patient's serum, a blue color develops. When the enzymatic reaction is stopped with 1N H2SO4, the contents of the wells turn yellow. The color, which is indicative of the concentration of antibody in the serum, can be read on a suitable spectrophotometer or ELISA microwell plate reader.

Certain limitations with the use of Sm test include: (1) the result of the assay should not be interpreted as being diagnostic. The results should only be used as an aid to diagnosis. The results should be interpreted in conjunction with the clinical evaluation of the patient. (2) Sera from patients with other autoimmune diseases and from normal individuals may contain auto antibodies. (3) The assay should be used only with serum. Icteric, lipemic, hemolyzed and heat inactivated serum should be avoided. (4) Index Values of â?¥ 10.00 should be reported as greater than 10.