Laboratory testing in autoimmune rheumatic diseases.

There are a number of pathological conditions in which tissue damage occurs in association with immune activation directed against components of normal tissue. The initial damaging events usually involve cells of the immune system, the T-cells, but the cell damage releases antigens that become targets for an antibody response. The detection and quantification of autoantibodies has become an important component in the diagnosis and management of autoimmune rheumatic diseases such as rheumatoid arthritis, systemic lupus erythematosus, the systemic vasculitides and systemic sclerosis. Each of these diseases is associated with a particular autoantibody or group of autoantibodies. They are usually detected by their reaction against tissue components using subjective methods such as indirect immunofluorescence. Any positive samples are further analysed using more specific and quantitative methods for the 'quantification' of the specific autoantibody concentration. It is important that these autoantibodies are not considered to be 'gold standard' tests: they are no more than markers of the disease with significant limitations. They are best used as part of a diagnostic panel rather than as a marker indicating one particular disease. Techniques are gradually improving, giving numerical results rather than titres, but a lack of standardization makes these results extremely variable. Many of the markers show no correlation with disease activity. Their use should be restricted to the initial investigation and not repeated every time the patient is followed up. Other markers do, however, correlate with disease activity and can be used to monitor disease. When investigating patients who have symptoms associated with autoimmune rheumatic diseases, analytes such as immunoglobulins, complement components and C-reactive protein may all be measured.