Imagine a simple blood test that could flag most kinds of cancers at the earliest, most curable stage. Liquid biopsies could, in theory, detect a tumor well before it could be found by touch, symptoms or imaging. It will take further giant, costly studies for pan-cancer liquid biopsies to prove their efficacy for early detection. Last year the FDA approved the first two such tests, which scan for tumor DNA so doctors can select mutation-targeted drugs. Scientists are working on blood tests to detect the first signs of cancer recurrence in patients who have completed treatment.