Sm and Sm/RNP Antibodies

Anti-Smith (Anti-Sm) antibodies are extractable nuclear antigen autoantibodies that are highly specific for systemic lupus erythematosus (SLE). They are less sensitive than ANA or anti-dsDNA, but when present they strongly support an SLE diagnosis and are commonly used as part of connective tissue disease serology panels.

Smith (Sm) and Ribonucleoprotein (RNP) antibodies are specific types of ENAs. Anti-Sm is highly specific for Systemic Lupus Erythematosus (SLE), though only present in ~20-30% of patients. Anti-RNP is characteristic of Mixed Connective Tissue Disease (MCTD) when present in high titers without other specific antibodies, but can also be found in SLE and other autoimmune conditions.