Positive predictive value (PPV)

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.

A blood test can find early, treatable cancers in people who feel well, including cancers for which no screening exists. It is not a replacement for mammography or colonoscopy but a possible addition. Larger randomised trials are needed to show benefit outweighs harm.

Lung screening works when it uses volumetric nodule management, and it works against a no-screening control. The protocol underpins the UK Targeted Lung Health Check programme and European recommendations. Benefit in women remains less precisely estimated.

For people with a heavy smoking history, an annual low-dose CT scan is one of the few screening tests proven to reduce cancer deaths. Most abnormal scans are not cancer, so screening must be paired with careful nodule management. It does not apply to never-smokers or light smokers.