Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (724 page)

   
Anti-Smith antibodies and anti-RNP antibodies

   The anti-Smith (anti-Sm) and anti-ribonucleoprotein (anti-RNP) systems are considered together, since they coexist in many patients with SLE and bind to related but distinct antigens.

   Anti-Sm antibodies occur more frequently in African Americans and Asians than in Caucasians with SLE.

   Anti-Sm antibodies generally remain positive when titers of anti-DNA antibodies have fallen into the normal range and clinical activity of SLE has waned. Therefore, measurement of anti-Sm titers may be useful diagnostically, particularly at a time when DNA antibodies are undetectable.

   Anti-RNP antibodies bind to antigens that are different from but related to Sm antigens. These antibodies bind to proteins containing only U1-RNA. Anti-RNP antibodies are found in 3–69% of patients with SLE but are a defining feature in the related syndrome, MCTD. The antibody is present in lower titers in several other rheumatic diseases, including primary Raynaud phenomenon, RA, and scleroderma.

   
Ro/SSA and La/SSB antibodies

   Anti-Ro/SSA and anti-La/SSB antibodies have been detected with high frequency in patients with Sjögren syndrome. They also have diagnostic usefulness in patients with SLE. They are infrequently seen in other connective tissue diseases such as scleroderma, polymyositis, MCTD, and RA.

   Anti-Ro/SSA antibodies have been associated with photosensitivity, a rash known as subacute cutaneous lupus, cutaneous vasculitis (palpable purpura), interstitial lung disease, neonatal lupus, and congenital heart block connective tissue disease. A minority evolve into a well-defined disorder.

   Anti-La/SSB antibodies are found in the following circumstances:

   It is very unusual to encounter sera that contain anti-La/SSB activity without demonstrable antibodies to Ro/SSA in patients with SLE or Sjögren syndrome.

   Isolated anti-La/SSB antibody activity has been seen in some patients with primary biliary cirrhosis and autoimmune hepatitis.

   Antibodies to the La/SSB antigen are present in 70–95% of patients with primary Sjögren syndrome, and in 10–35% of patients with SLE, and are occasionally seen in patients with cutaneous LE, scleroderma disorders, and RA.