ANA (1:80 titer) — Systemic Lupus Erythematosus
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 2% (sensitivity 98%) |
| False-positive rate | 25% (specificity 75%) |
| Bayes factor (positive test) | 5× |
| Bayes factor (negative test) | 1/50× |
| Base rate | 0.1% in the general adult population |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Unselected patient | 0.1% | 5 × 0.1% = 0.5%0.1%× 50.5% | 1/50 × 0.1% ≈ 0% |
| Young woman: malar rash, arthralgias, photosensitivity | 5% | 5 × 5% = 25%5%× 525% | 1/50 × 5% = 0.1%5%÷ 500.1% |
Low-titer positive (5×) is weak — most are false positives. The negative Bayes factor (1/50×) is extraordinary — a negative ANA essentially rules out SLE. See also ANA at higher titers (≥1:320) for a much stronger positive result.
- + result: risk only moves from appendicitis this year (0.1%) to identical twin (0.5%)
- − result: risk drops to essentially zero
Sources:
- Meta-regression, Arthritis Care Res, 2017. Wiley.
- AAFP. Antibody testing for SLE.
- PMC. Finding lupus in the ANA haystack.