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Artist's rendering of a cancerous cell (Creative Commons)

Population screening programmes are a key part of our public health arsenal, saving an estimated 10,000 lives in England each year. But it is an unusual form of healthcare in that otherwise healthy individuals are contacted before symptoms arise. Additionally, despite their potential to prevent disease, such programmes, if not properly designed, can harm as well as help the population.

One of the key challenges with developing new programmes is around test specificity: how likely it is that a person will be told they have a disease if they don’t actually have it (a “false positive”). High rates of false positives can lead to costly, invasive follow-ups, and significant anxiety for individuals.

These will be key considerations for UK regulators in their upcoming evaluation of the Galleri Test, a potentially revolutionary new blood test for the early identification of cancer. Preliminary studies suggest it is able to detect more than 50 different types of cancer before symptoms appear, and locate with 90 per cent accuracy the tissue in which the tumour originated. Crucially, the test is quick, easy and non-invasive, and the false positive rate appears to be comparatively low.

The test works by looking for tiny bits of DNA that have broken off tumour cells and float around in the bloodstream (“cell-free DNA”). It is currently available in the US for those at high risk. However, its power as a screening tool – identifying early cancer in symptomless individuals not at high risk, who therefore have the best chance of successful treatment – has not yet been assessed. Since September, Galleri has been rolled out in a large NHS clinical trial involving 140,000 healthy volunteers. The trial is the biggest of its kind, with results expected in 2023. If successful, it has the potential to expand to a million more volunteers from 2024.

Some have questioned whether the test will deliver on the hype when used in a real-world setting, with scientists all too aware of past disappointments. Hope that it is the holy grail of cancer screening should be tempered; its success remains to be seen.

This piece is a preview from the Witness section of New Humanist winter 2021. Subscribe today.