A blood test that can detect several types of cancer before a person falls ill has been hailed as a “holy grail” by many newspapers, but to be useful it will need to be refined to become much more accurate, otherwise it could fail to detect large numbers of cancer cases.

In a study of 749 people without cancer, and 878 people who had been newly diagnosed with cancer, the test detected 90 per cent of cases of ovarian cancer, and 80 …