Multi cancer detection tests or MCDs can detect many cancers through a simple blood test. Many detect fragments of cancerous DNA that have broken off a tumour and are circulating in the blood. They can often then identify where the cancer may be. The NHS is currently involved in the words largest trial of one such test. The Galleri test is made by US firm Grail and the company says its mission is to detect cancer early when it can be cured. Heavyweight investors include Jeff Bezos and Bill Gates. More than 140,000 volunteers took part in the trial and the results will be known next year. If successful, the test may be rolled out further. File on 4 Investigates looks at how effective such tests are and whether the trial will show it can save lives. Medical journalist Deb Cohen speaks to one woman whose cancer was found by the test and successfully treated. But she also hears from firefighters in America - where it's available to the public - who say it missed some cancers and some people who got a positive result were found not to have the disease after undergoing diagnostic procedures - a so called false positive. With many hospitals trusts missing cancer treatment targets, what is the best way forward for the NHS to improve outcomes Reporter: Deb Cohen Producer: Paul Grant Technical producer: Craig Boardman Production co-ordinator: Tim Fernley Editor: Tara McDermott