Tests for Autoimmunity Issues
When certain autoantibodies are present, this provides valuable diagnostic information for SLE. The most specific tests are those that detect high levels of these autoantibodies. The most common and specific tests for autoantibodies and other elements of the immune system are listed first.
- Antinuclear Antibody (ANA) – A screening test for ANA is standard in assessing SLE because it is positive in close to 100 percent of patients with active SLE. However, it is also positive in 95 percent of patients with mixed connective tissue disease, in more than 90 percent of patients with systemic sclerosis, in 70 percent of patients with primary Sjögren’s syndrome, in 40 to 50 percent of patients with rheumatoid arthritis, and in 5 to 10 percent of patients with no systemic rheumatic disease. Patients with SLE tend to have high titers of ANA. False-positive results are found during the course of chronic infectious diseases, such as subacute bacterial endocarditis, tuberculosis, hepatitis, and malaria. The sensitivity and specificity of ANA determinations depend on the technique used.
- Anti-Sm – Anti-Sm is an immunoglobulin specific against Sm, a ribonucleoprotein found in the cell nucleus. This test is highly specific for SLE; it is rarely found in patients with other rheumatic diseases. However, only 30 percent of patients with SLE have a positive anti-Sm test.
- Anti-dsDNA – Anti-dsDNA is an immunoglobulin specific against native (double-stranded) DNA. This test is highly specific for SLE; it is not found in patients with other rheumatic diseases. Fifty percent of patients with active SLE have a positive anti-dsDNA test. For many patients with anti-dsDNA, the titer is a useful measure of disease activity. The presence of antidsDNA is associated with a greater risk of lupus nephritis.
- Anti-Ro(SSA) and Anti-La(SSB) – These immunoglobulins, commonly found together, are specific against RNA proteins. Anti-Ro is found in 30 percent of SLE patients and 70 percent of patients with primary Sjögren’s syndrome. Anti-La is found in 15 percent of people with lupus and 60 percent of patients with primary Sjögren’s syndrome. Anti-Ro is highly associated with photosensitivity; both are associated with neonatal lupus.
- Complement Proteins – Complement proteins constitute a serum enzyme system that helps mediate inflammation. Complement components are triggered into an activated form by such immunologic events as interaction with immune complexes. Complement components are identified by numbers (C1, C2, etc.). Genetic deficiencies of C1q, C2, and C4, although rare, are commonly associated with SLE. A test to evaluate the entire complement system is called CH50. The most commonly measured complement components are the serum levels of C3 and C4.
- Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP) – Tests for ESR and CRP are nonspecific tests to detect generalized inflammation. Levels are generally increased in patients with active lupus and decline when corticosteroids or nonsteroidal anti-inflammatory drugs are used to reduce inflammation. However, they do not directly reflect disease activity.
- Antiphospholipid Antibodies (APLs) – APLs are autoantibodies that react with phospholipids. Recent data indicate that APLs recognize a number of phospholipid-binding plasma proteins (e.g., prothrombin, ß2 glycoprotein I) or protein-phospholipid complexes rather than phospholipids alone. APLs are present in 50 percent of people with lupus. Antiphospholipid syndrome occurs in 50 percent of SLE patients who have the lupus anticoagulant. This syndrome is characterized by a persistently positive lupus anticoagulant or medium-to-high titer anticardiolipin or anti-ß2 glycoprotein I in the clinical setting of thrombosis, fetal loss, multiple first-trimester losses, or preterm birth from severe placental vasculopathy.