Scientists said their findings, published in the journal Annals of Oncology, show that the test accurately detects cancer often before any signs or symptoms appear, while having a very low false positive rate. The test, developed by US-based company Grail, looks for chemical changes in fragments of genetic code – cell-free DNA (cfDNA) – that leak from tumours into the bloodstream. The Guardian first reported on the test last year and how it had been developed using a machine learning algorithm – a type of artificial intelligence. It works by examining the DNA that is shed by tumours and found circulating in the blood. More specifically, it focuses on chemical changes to this DNA, known as methylation patterns.