A blood test to detect cancer? Some patients are using them already.

When her husband was undergoing cancer treatment, Cindy Perez of Southwest Ranches, Fla., learned about a new blood test that could help find early cancers. The 50-year-old said she felt fine, but her husband urged her to take the test anyway. To her surprise, the blood test — called Galleri — came back positive. Scans revealed a small tumor in her groin and a diagnosis of mantle cell lymphoma, a rare but aggressive form of cancer.