Emerging Screening — Galleri, ctDNA
Liquid biopsy is rewriting screening
Until recently, cancer screening was organ-by-organ. Multi-cancer early detection (MCED) tests look for cancer-signal DNA fragments from any tissue, from a single blood draw. The underlying tech (methylation + fragmentation patterns on cfDNA) is genuinely new.
Galleri — PATHFINDER results
Grail's Galleri is the best-studied MCED test. In the 6,621-person PATHFINDER real-world study, Galleri detected cancer signals in 1.4% of participants (92 true positives), with a positive predictive value of 38% for screening-eligible vs 69% for non-screening-eligible cancers. Specificity was 99.1% (~0.9% false-positive rate). $949 cash.
The 'what if it's positive' problem
A positive Galleri test tells you "cancer signal detected" and points to a likely origin tissue. It does NOT tell you which cancer, stage, or whether it's actionable. Workup requires imaging (often whole-body MRI or PET) — costly, sometimes inconclusive, occasionally revealing aggressive cancer that otherwise would have been missed. Know your willingness to follow up before testing.
Who benefits most
Highest value: high-risk individuals (strong family history, BRCA+, Lynch syndrome) or people with past cancer watching for recurrence. Average-risk adults under 50 see smaller absolute benefit because cancer rates are lower. NHS and Medicare pilots are ongoing; coverage will likely expand with more positive data.
Lead-time bias — earlier ≠ better
MCED tests will detect cancers earlier than they'd otherwise be found — but "earlier detection" doesn't always mean "longer life". Some cancers caught early would never have caused symptoms (overdiagnosis); others get caught earlier without changing the timing of death (lead-time bias). The honest version: until randomised mortality trials report (NHS-Galleri trial reads out ~2027), we don't yet know which fraction of detected signals translate into lives saved. Worth running in if you'd act on a positive; less obviously worth it if a positive would just trigger anxiety + workups without changing your behaviour.
Key Takeaway
MCED tests like Galleri are real, not marketing. They add something standard screening doesn't — a single-draw look at a very wide range of cancers. Lead-time bias + overdiagnosis are still real concerns, and the mortality data isn't in yet. For high-risk individuals, they're increasingly worth it. For average-risk adults, they complement (not replace) standard screening.