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Diagnostic Tests-ANA, Anti–Double-Stranded DNA (anti-dsDNA), and Anti-Smith (anti-Sm) Antibodies
1.
What diseases are they associated with?
- ANA (Antinuclear Antibody)
- More than 98% sensitive for systemic lupus erythematosus (SLE)
- Not specific for SLE (can be positive in other autoimmune diseases and even in healthy individuals)
- A negative ANA essentially excludes SLE, with about a 98% negative predictive value
- Anti–double-stranded DNA (anti-dsDNA)
- Approximately 99% specific for SLE
- Levels often correlate with disease activity, particularly lupus nephritis
- Anti-Smith (anti-Sm)
- Extremely specific for SLE
- Less sensitive, but when present, it is diagnostic of lupus
2.
When do you answer/order this test?
Answer ANA, anti-dsDNA, and anti-Sm antibodies when the clinical presentation suggests systemic lupus erythematosus, classically:
- Young female with:
- Multiple spontaneous abortions (suggesting lupus anticoagulant / antiphospholipid antibodies)
- Rash (malar or photosensitive)
- Joint pains / inflammatory arthritis
- May also have renal disease, cytopenias, or serositis
- ANA → screening test (rules out SLE if negative)
- Anti-dsDNA & Anti-Sm → confirmatory tests (rule in SLE if positive)
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