Laboratory Tests Used to Diagnose and Evaluate SLE

Measurements of Autoimmunity continued...

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% of patients with SLE have a positive anti-Sm test.

Anti-nDNA

Anti-nDNA 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. Sixty to eighty percent of patients with active SLE have a positive anti-nDNA test. For many patients with anti-nDNA, the titer is a useful measure of disease activity. The presence of anti-nDNA 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% of SLE patients and 70% of patients with primary Sjögren's syndrome. Anti-La is found in 15% of lupus patients and 60% of patients with primary Sjögren's syndrome. Anti-Ro is highly associated with photosensitivity; both are associated with neonatal lupus.

Complement

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 level of C3 and C4. These tests are particularly useful in evaluating kidney involvement and in monitoring the disease over time.

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 NSAIDs are used to reduce inflammation.

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 30-40% of lupus patients. A positive APL test plus the presence of arterial and venous thrombosis and thrombo- embolism or recurrent fetal deaths or thrombocytopenia is called APL syndrome. APL syndrome affects about a third of lupus patients with APLs (10-15% of all lupus patients). APLs and APL syndrome may also occur in patients without lupus (primary APL syndrome). APLs are detected in the following three types of laboratory assays.

Syphilis Serology

Certain blood tests for syphilis may be falsely positive in lupus patients. Chronically false-positive VDRL or rapid plasma reagin (RPR) tests may occur in patients with lupus. Cardiolipin, a phospholipid, is a component of the antigenic mixture used in these assays. More specific tests for syphilis, such as the fluorescent treponemal antibody-absorbed (FTA-ABS) and microhemagglutination-Treponema pallidum (MHA-TP) assays, are almost always negative in lupus patients without syphilis.