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Positive predictive value (PPV) is the chance that a positive test result is actually right: true positives divided by all positives. Because it falls as a disease becomes rarer, even a specific screening test gives mostly false alarms when prevalence is low, which is why PPV decides whether a multi-cancer blood test is useful.
Positive predictive value (PPV) is the probability that a test result saying 'cancer' is actually correct. It is true positives divided by all positives, so it depends heavily on how common the disease is in the tested population: even a highly specific screening test yields many false alarms when prevalence is low. This makes PPV central to judging Multi-cancer early detection (MCED) tests such as Galleri, and it recurs in the PATHFINDER 2 trial, the early detection roadmap, and the bottlenecks on cancers found late and on overdiagnosis and false alarms. It is also cited by Breath and volatile-organic-compound detection, Harbinger Health, and ideas on a national pathway for people with a positive multi-cancer blood test and urine DNA triage for haematuria.
Showing the technology this term belongs to: Multi-cancer early detection (MCED).
These are the numbers GRAIL's US application rests on. Six in ten positive results were cancer and the predicted organ was usually right, but six in ten cancers diagnosed in the year were not flagged, and the study had no control group, so it cannot say whether testing changes outcomes.
For someone offered the test, these are the numbers that describe what a result means in an NHS population: about 1 in 100 tests came back positive each year, between 46 and 58 in 100 positives were cancer, and a negative result left about 1 in 100 with an undetected cancer within the year. The test found between a quarter and a third of all cancers diagnosed in a screening round, and about half to two thirds of the 12 cancer types it was designed to find. Whether finding them this way reduces late-stage diagnoses is the primary question, and this paper says only that the answer was no; the primary-endpoint paper had not appeared on Europe PMC by 23 September 2026.
A prospective screening result in a lower-middle-income country, with a positive predictive value similar to PATHFINDER's; the sensitivity rests on 24 cancers, and the test's tissue call was right about half the time.
Outcomes are known for fewer than half of the positives, so the share of positives that were cancer in this group cannot be read as the test's positive predictive value, and cancers missed by the test are not counted at all.
In people who already have symptoms the test is more often right when positive, because cancer is commoner, but it still misses most stage I cancers, so a negative result cannot cancel an urgent referral.
A multi-cancer blood test can be run in ordinary clinics, and most positive results can be resolved with imaging. But six in ten positives are false alarms that take months to resolve, and the test misses most cancers. Whether it reduces late-stage cancer or deaths is unknown; that requires randomised trials.
NHS-Galleri is the trial that will decide whether a blood test for many cancers at once should be offered by a health system. Its endpoint is a reduction in late-stage cancer rather than deaths, so even a positive result leaves the mortality question to be settled by longer follow-up.
Fixes the window in which a blood test could plausibly work and shows why CA 19-9 alone is not enough: half of early cases are missed even at diagnosis, and Lewis-negative patients are missed entirely.
Shares Real-world data and clinical experience from over 100,000 multi-cancer early detection tests, Multi-cancer blood test as the first step for patients with non-specific symptoms, Multi-cancer early detection test in symptomatic patients referred for cancer investigation in England and Wales (SYMPLIFY): a large-scale, observational cohort study, The PATHFINDER Study: Assessment of the Implementation of an Investigational Multi-Cancer Early Detection Test into Clinical Practice.
Shares Breath and volatile-organic-compound detection, CCGA: the Circulating Cell-free Genome Atlas behind the Galleri methylation test, New-onset diabetes as a signal of pancreatic cancer (and the ENDPAC score), Vanguard Study (NCI Cancer Screening Research Network).
Shares Real-world data and clinical experience from over 100,000 multi-cancer early detection tests, Multi-cancer blood test as the first step for patients with non-specific symptoms, Multi-cancer early detection test in symptomatic patients referred for cancer investigation in England and Wales (SYMPLIFY): a large-scale, observational cohort study, The PATHFINDER Study: Assessment of the Implementation of an Investigational Multi-Cancer Early Detection Test into Clinical Practice.
Shares Shield MCD (Shield Multi-Cancer Detection), Cologuard, Multitarget stool RNA test (ColoSense), Cancerguard.
Shares Multi-cancer early detection test in symptomatic patients referred for cancer investigation in England and Wales (SYMPLIFY): a large-scale, observational cohort study, Shield MCD (Shield Multi-Cancer Detection), CCGA: the Circulating Cell-free Genome Atlas behind the Galleri methylation test, Performance and safety of a multi-cancer early detection test: the PATHFINDER 2 study.
Shares Lewis-negative (Lewis antigen-negative, CA 19-9 non-secretor) status, Blood-based pancreatic cancer detection in new-onset diabetes, New-onset diabetes as a signal of pancreatic cancer (and the ENDPAC score), Run the ENDPAC score on every new diabetes diagnosis after 50 and scan the high scorers.
Shares Cologuard, Multitarget stool DNA testing for colorectal-cancer screening, Multitarget stool RNA test (ColoSense), Cancerguard.
Shares A urine DNA test to decide who with blood in the urine needs a camera test, Harbinger Health, CCGA: the Circulating Cell-free Genome Atlas behind the Galleri methylation test, Cologuard.