BOSTON (AP) - A blood test so sensitive that it can spot a single cancer cell lurking among a billion healthy ones is moving one step closer to being available at your doctor's office.

Boston scientists who invented the test and health care giant Johnson & Johnson announced Monday that they are joining forces to bring it to market. Four big cancer centers also will start studies using the experimental test this year.

Stray cancer cells in the blood mean that a tumor has spread or is likely to, many doctors believe. A test that can capture such cells has the potential to transform care for many types of cancer, especially breast, prostate, colon and lung.

Initially, doctors want to use the test to try to predict what treatments would be best for each patient's tumor and find out quickly if they are working.

"This is like a liquid biopsy" that avoids painful tissue sampling and may give a better way to monitor patients than periodic imaging scans, said Dr. Daniel Haber, chief of Massachusetts General Hospital's cancer center and one of the test's inventors.

Ultimately, the test may offer a way to screen for cancer besides the mammograms, colonoscopies and other less-than-ideal methods used now.