Background

Systemic rheumatic diseases are autoimmune disorders such as systemic lupus erythematosus (SLE), polymyositis, Sj?gren'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.

The La (SS-B) antigen is an RNA-protein complex; antibodies against it are found in Sj?gren's syndrome (40%) and SLE (15%). Anti-La (SS-B) antibodies have also been detected in patients with systemic sclerosis and neonatal lupus erythematosus. There is evidence associating these antibodies to particular clinical characteristics, and the observation that patients may have autoantibodies before the full onset of disease confirms the need for an accurate and easy-to-use test.

Technical information

The wells of the microtitre strips are coated with affinity-purified La (SS-B) 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-La (SS-B) autoantibody can be estimated by interpolation from a dose?response curve based on Standards.