Smith (Sm) Antibodies, IgG, Serum

Casandra

Casandra Test Code MC56566Version 1 (DRAFT)

Performing Lab

Lab Mayo Clinic LabLab Test ID SMCPT Code 86235

Smith (Sm) Antibodies, IgG, Serum

Clinical Use

Use

Evaluating patients with clinical features suggestive of antinuclear antibody associated connective tissue disease and the confirmation of a diagnosis of systemic lupus erythematosus. Testing for Smith antibodies is not useful in patients without demonstrable antinuclear antibodies. The presence of antibodies to Sm is specific for SLE with a sensitivity of 5% to 30%. Based on classification criteria for SLE, patients positive for anti-Sm antibodies already fulfil 60% of the criteria required for SLE classification. However, anti-Sm antibodies may occur together with ribonucleoprotein antibodies in certain ANA-associated connective tissue diseases. In routine clinical practice, a variety of immunoassays are used for the determination of anti-Sm antibodies.

Special Instructions

Complete a Kidney Transplant Test Request form if not ordering electronically. Refer to Connective Tissue Disease Cascade and First-Line Screening for Autoimmune Liver Disease Algorithm for additional test situations including reflex and additional tests.

Limitations

The presence of antibodies to Sm is specific for SLE but has limited sensitivity, observed in only 5% to 30% of the cases. Immunoassays for detecting anti-Sm antibodies differ in their analytical characteristics, which may affect diagnostic performance. It is also noted that there might be ethnic variability in the prevalence of positive anti-Sm antibody. The solid-phase immunoassays might include a mixture of Sm antigens that can affect the results depending on their composition.

Test Details

FDA Approved