Anti-Sm Ab (RDL)
Code:16005|CPT:86235|LOINC:11090-8
| Includes | Anti-Sm Ab (RDL) |
|---|
Analysis details
Methodology
- Enzyme immunoassay (EIA)
Expected Turnaround Time
1 day
Special Instructions
- Avoid smoking for at least 30 minutes before the blood draw.
How to use
The Anti-Smith antibody (anti-Sm) test serves as a highly specific serologic marker for systemic lupus erythematosus, with limited sensitivity (approximately 20%–30% of SLE cases). In the setting of a positive antinuclear antibody (ANA) screen, anti-Sm results help substantiate a diagnosis of SLE and refine the differential among systemic autoimmune connective tissue diseases. Anti-Sm reactivity is uncommon outside SLE, with only occasional reports in monoclonal gammopathies and uveitis. The assay is commonly considered alongside extractable nuclear antigen (ENA) antibodies and other ANA-specific antibodies to contextualize serologic patterns.
Limitations
Anti-Sm (Smith) antibodies recognize antigens associated with U1, U2, and U4 small nuclear ribonucleoproteins. This specificity underpins their strong association with systemic lupus erythematosus and their infrequency in other systemic autoimmune diseases. Anti-Sm status contributes to SLE classification frameworks. Antibody concentrations do not correlate with systemic lupus erythematosus disease activity and are not suitable for monitoring disease course. Accordingly, anti-Sm testing is used for diagnostic classification and differential diagnosis rather than for serial disease tracking.
| Reference interval |
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|---|---|---|---|---|---|
| Indications | Positive ANA by HEp-2 indirect immunofluorescence or an equivalent screening method., Clinical picture consistent with a systemic autoimmune connective tissue disorder (for example, photosensitivity, inflammatory arthritis, Raynaud phenomenon, leukopenia, autoimmune hemolytic anemia, persistent fever, weight loss, or myalgia)., Clarifying the differential diagnosis when systemic lupus erythematosus or overlap syndromes are suspected., Follow-up serologic assessment in patients with known or suspected systemic connective tissue disease. |
Specimen Requirements
| Specimen | Serum |
|---|---|
| Container | Gold/Tiger Top (SST, Gel Separator) |
| Volume | 1 mL (min 0.3 mL) |
| Storage Instructions | Room temperature, Refrigerated, Frozen |
References
Fischbach F.T., Dunning M.B. A Manual of Laboratory and Diagnostic Tests. 8th ed. Lippincott Williams & Wilkins; 2008:1344.
Wilson D. McGraw-Hill Manual of Laboratory and Diagnostic Tests. 1st ed. McGraw-Hill; 2007:666.
Sabatine M. Pocket Medicine. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2008.