Lumipulse pTau217/Aβ42 Ratio — Alzheimer's Disease
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 1% (sensitivity 99%) |
| False-positive rate | 15% (specificity 85%) |
| Bayes factor (positive test) | 5× |
| Bayes factor (negative test) | 1/100× |
| Base rate | 10% of adults over 65 presenting with memory concerns |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Adult over 65 with memory concerns | 10% | 5 × 10% = 50%10%× 550% | 1/100 × 10% = 0.1%10%÷ 1000.1% |
| Strong family history, progressive decline | 25% | 5 × 25% ≥ 100%25%× 5100% | 1/100 × 25% = 0.25%25%÷ 1000.25% |
5 × 25% ≥ 100%25%× 5100%: exact posterior is 69%. First FDA-cleared blood test for diagnosing Alzheimer's (May 2025). Measures a two-marker ratio (pTau217 and β-amyloid 1-42). The FDA-cleared cutpoints prioritize sensitivity (99%) — don't miss anyone — at the cost of lower specificity (85%). A positive needs confirmation with amyloid PET or CSF. The negative BF (1/100×) is extraordinary — a negative result essentially rules out amyloid pathology. Available through Labcorp. For adults 55+ with cognitive symptoms. Not currently covered by Medicare or most insurance.
- + result: risk goes from left-handed (10%) to coin flip (50%) — needs confirmation
- − result: risk drops to appendicitis this year (0.1%) — essentially rules it out
Sources:
- FDA. First blood test for Alzheimer's, May 2025.
- Alzheimer's & Dementia, 2025. Diagnostic accuracy of pTau217/Aβ42.
- Labcorp. pTau217/Aβ42 ratio test.