A Blood Test for Early Cancer Detection Sparks Debate A new ‘liquid biopsy’ detects bits of DNA common in several cancers, though critics question its value Scientists have long dreamed of spotting cancers with a simple blood test in people who haven't shown symptoms. One company says that day is here—though it has yet to convince government regulators. Pathway Genomics began marketing a blood test in mid-September that it says can detect DNA fragments linked to 10 common can