ANA (Antinuclear Antibody) Test by Indirect Immunofluorescence on HEp-2 Cells
The antinuclear antibody (ANA) test using indirect immunofluorescence on HEp-2 cells is the primary screening blood test for lupus, with a sensitivity approaching 100% for systemic lupus erythematosus. 1, 2
Primary Screening Test
The ANA test by indirect immunofluorescence assay (IIFA) on HEp-2 cells remains the gold standard screening method 1. This test is positive in nearly every case of SLE, making it an excellent "rule-out" test 2. A negative ANA (titer <1:40) essentially excludes lupus in most cases 3.
Important Caveats About ANA Testing
The ANA test is highly sensitive but not specific - it can be positive in many other conditions and even in healthy individuals 2, 3. This means:
- A positive ANA alone does NOT diagnose lupus
- The test must be interpreted in the appropriate clinical context
- False positives are common, especially in primary care settings 4
When to Order ANA Testing
Order ANA testing only when there is clinical suspicion based on:
- Unexplained multisystem inflammatory disease
- Symmetric joint pain with inflammatory features
- Photosensitive rash
- Unexplained cytopenias
- Unexplained involvement of two or more organ systems 2, 3
Do NOT order ANA for nonspecific symptoms like isolated fatigue or malaise - the test has limited value in these situations 2.
Follow-Up Testing After Positive ANA
If ANA is positive (≥1:40) with characteristic clinical features, proceed with more specific antibody testing 3:
- Anti-dsDNA antibodies: Highly specific for SLE (present in 40-80% of patients). Use Farr assay or Crithidia luciliae immunofluorescence test (CLIFT) for best specificity 1, 5
- Anti-Sm antibodies: Very specific for SLE 3
- Anti-ENA panel (extractable nuclear antigens): Helps identify specific lupus subsets 1
- Anti-phospholipid antibodies: Important for identifying antiphospholipid syndrome risk 5
Critical Exception
In cases of high clinical suspicion, request specific antibody testing (anti-dsDNA, anti-Sm, anti-Ro/SSA, anti-ribosomal P) even if the ANA is negative, as some patients with lupus can be ANA-negative or have antibodies that are missed by standard ANA screening 1, 3.