Scientists have the first major evidence that blood tests called liquid biopsies hold promise for screening people for cancer. Hong Kong doctors tried it for a type of head and neck cancer, and boosted early detection and one measure of survival.

The tests detect DNA that tumors shed into the blood. Some are used now to monitor cancer patients, and many companies are trying to develop versions of these for screening, as possible alternatives to mammograms, colonoscopies and other such tests. The new study shows this approach can work, at least for this one form of cancer and in a country where...