ANA (≥1:320 titer) — Systemic Lupus Erythematosus
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 2% (sensitivity 98%) |
| False-positive rate | 5% (specificity 95%) |
| Bayes factor (positive test) | 20× |
| 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% | 20 × 0.1% = 2%0.1%× 202% | 1/50 × 0.1% ≈ 0% |
| Young woman: malar rash, arthralgias, photosensitivity | 5% | 20 × 5% ≥ 100%5%× 20100% | 1/50 × 5% = 0.1%5%÷ 500.1% |
20 × 5% ≥ 100%5%× 20100%: exact posterior is 51%. High titer is a much stronger positive signal than 1:80. Multiple autoantibodies (≥3 positive: ~50×) are even more informative. The negative Bayes factor (1/50×) is extraordinary — a negative ANA essentially rules out SLE.
- + result: risk goes from appendicitis this year (0.1%) to red hair (2%)
- − 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.