Elecsys pTau181 — Alzheimer's Disease
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 16% (sensitivity 84%) |
| False-positive rate | 22% (specificity 78%) |
| Bayes factor (positive test) | 3× |
| Bayes factor (negative test) | 1/5× |
| Base rate | 10% of adults over 65 presenting with memory concerns |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Adult over 65 with memory concerns | 10% | 3 × 10% = 30%10%× 330% | 1/5 × 10% = 2%10%÷ 52% |
| Strong family history, progressive decline | 25% | 3 × 25% = 75%25%× 375% | 1/5 × 25% = 5%25%÷ 55% |
The only FDA-cleared blood test for primary care. Designed as a rule-out test — a negative result has 97.9% negative predictive value in low-prevalence primary care populations. The positive result is weak (3×) because the test is optimized for sensitivity over specificity. A positive pTau181 warrants referral for confirmatory testing (amyloid PET or CSF biomarkers). Single marker (pTau181 only), available through Labcorp. Not currently covered by Medicare or most insurance.
- + result: risk goes from left-handed (10%) to 30% — needs confirmation
- − result: risk drops to red hair (2%) — strong reassurance
Sources:
- Roche. Elecsys pTau181 FDA clearance.
- Alzheimer's Association. FDA clearance statement.