A blood test so sensitive that it can spot a single cancer cell lurking among a billion healthy ones took a giant step closer today.

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

Stray cancer cells in the blood mean that a tumour 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 tumour 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 centre 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.

"There's a lot of potential here and that's why there's a lot of excitement," said Dr Mark Kris, lung cancer chief at Memorial Sloan-Kettering Cancer Centre in New York. He had no role in developing the test, but Sloan-Kettering is one of the sites that will study it this year.

Many people have their cancers diagnosed through needle biopsies, which often do not provide enough of a sample to determine what genes or pathways control a tumour's growth, or the sample may no longer be available by the time the patient sees a specialist to decide what treatment to prescribe.

Doctors typically give a drug or radiation treatment and then do a CT scan two months later to look for tumour shrinkage.

Some patients only live long enough to try one or two treatments, so a test that can gauge success sooner, by looking at cancer cells in the blood, could give patients more options.

"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," Dr Haber said.

The only test on the market now to find tumour cells in blood - CellSearch, made by J&J's Veridex unit - just gives a cell count and does not capture whole cells that doctors can analyse to choose treatments.

Interest in trying to collect these cells soared in 2007 after Dr Haber and his colleagues published a study of Massachusetts General's test.

It is far more powerful than CellSearch and traps cells intact, requiring only a couple of teaspoons of blood and can be done repeatedly to monitor treatment or determine why a drug has stopped working and what to try next.

Dr Kris said doctors could give a drug one day and sample blood the next day to see if the circulating tumour cells were gone.

The test uses a microchip that resembles a lab slide covered in 78,000 tiny posts, like bristles on a hairbrush. The posts are coated with antibodies that bind to tumour cells. When blood is forced across the chip, cells ping off the posts like balls in a pinball machine. The cancer cells stick and stains make them glow so researchers can count and capture them for study.

The test can find one cancer cell in a billion or more healthy cells, said Mehmet Toner, a Harvard University bioengineer who helped design it.

Studies of the chip have been published in the journals Nature, the New England Journal of Medicine and Science Translational Medicine. It is the most promising of several dozen that companies and universities are rushing to develop to capture circulating tumour cells, said Bob McCormack, technology chief for Veridex.

The agreement announced today will have Veridex and J&J's Ortho Biotech Oncology unit work to improve the microchip, including trying a cheaper plastic to make it practical for mass production. No price goal has been set, a company official said, but the current CellSearch test costs several hundred dollars.

Already, scientists have been surprised to find that more cancer patients harbour these stray cells than has been believed. In one study, the test was used on men thought to have cancer confined to the prostate, "but we found these cells in two-thirds of patients", Mr Toner said.

This might mean that cancer cells enter the blood soon after a tumour starts, or that more cancers have already spread but are unseen by doctors.

Dr Minetta Liu, a breast cancer specialist at Georgetown University's Lombardi Comprehensive Cancer Centre, who has been a paid speaker for Veridex, hopes the cells will one day aid cancer screening.

"The dream is, a woman comes in for her mammogram and gets a tube of blood drawn", so doctors can look for cancer cells in her blood as well as tumours on the imaging exam, she said.