A new blood test could transform the way doctors detect and treat many types of cancer.

The test, invented by Boston scientists and health care giant Johnson & Johnson, is so sensitive it can spot a single cancer cell among a billion healthy ones.

Doctors hope the new development will eventually help in determining what treatments would be best for a particular tumor and how quickly treatment is working.

"If you could find out quickly, 'this drug is working, stay on it,' or 'this drug is not working, try something else,' that would be huge," said Dr. Daniel Haber, chief of Massachusetts General Hospital's cancer center and one of the test's inventors.

Researchers say the test may also offer alternative ways to screen for cancer besides the mammograms, colonoscopies, and other less-than-ideal methods used currently.