Laboratory Testing in the Rheumatic Diseases
The laboratory evaluation for the rheumatic diseases involves both nonspecific and specific biomarkers to determine the extent of tissue injury, the presence of inflammation and autoimmunity, and serologic evidence for the diagnosis. Nonspecific markers of inflammation include C-reactive protein and the erythrocyte sedimentation rate. Specific markers differ by disease and include disease-related and disease-specific autoantibodies. Although common in rheumatoid arthritis, a positive rheumatoid factor can occur in other rheumatic and nonrheumatic diseases. More specific markers of rheumatoid arthritis are antibodies to citrullinated protein antigens (ACPAs). Antibodies to nuclear antigens (ANAs), which are a feature of many rheumatic diseases, target nucleic acids, nuclear proteins, and complexes of nucleic acids and proteins. The target antigens of ANAs vary by disease and include cytoplasmic as well as nuclear molecules. The pattern of ANA expression provides information for classification, diagnosis, and activity of systemic lupus erythematosus. ANAs are also present in diseases such as Sjögren syndrome, systemic sclerosis, and the idiopathic inflammatory myopathies. In the antiphospholipid antibody syndrome, antibodies to phospholipids and a protein called β2-glycoprotein1 can promote thrombosis in vivo, although they can inhibit blood clotting in vitro. For immune complex-mediated diseases, including systemic lupus erythematosus, measurement of complement proteins is useful for assessing disease activity. A group of antibodies, known as antineutrophil cytoplasmic antibodies (ANCAs), react with neutrophil proteins such as proteinase-3 and myeloperoxidase and are important markers of vasculitis. Depending on the clinical scenario, laboratory tests can diagnose and stage disease, help assess the risk for progressive disease, monitor the effects of treatment, and potentially identify appropriate therapy.
