A simple blood test could help doctors pinpoint more effective personalised treatments for men with advanced prostate cancer.
The test, which costs less than STG50 ($A87), can predict which patients are likely to respond to new targeted drugs and who might be better served by alternative therapies.
It looks for multiple copies of a gene for the androgen receptor, a hormone-sensitive molecule that helps many prostate cancers to grow.
Men with multiple copies of the gene were found to respond much less well to the drugs abiraterone and enzalutamide, which are used to treat advanced prostate cancer.
The drugs are given to men whose cancer is no longer responding to traditional hormone therapy and has started to spread.
"Some men can take these drugs for years without seeing a return of their cancer," said lead researcher Dr Gerhardt Attard, of London's Centre for Evolution and Cancer at The Institute of Cancer Research.
"But in other men, these drugs do not work well and the disease rapidly returns."
At present, there was no approved test to help doctors choose whether these were the best treatments for an individual, Attard said.
"Our method costs less than STG50, is quick to provide results and can be implemented in hospital laboratories across the NHS," he said.
"We are now looking to assess our test in prospective clinical trials and would hope it can become part of standard patient care."
Dr Iain Frame, director of research at Prostate Cancer UK, which part-funded the study, said a move away from a one-size-fits-all approach to treatment was needed.
"This test could be a significant step towards that and we'll be watching its development very closely," he said.
Scientists took blood samples from patients taking part in three different clinical trials for the study, published in the journal Annals of Oncology.
In the primary trial of 171 patients, men found to have multiple copies of the androgen receptor gene were four times more likely to die than those who had a negative test result.
Men with multiple copies of the gene from a second group of 94 patients responded to abiraterone and enzalutamide for an eightfold shorter time than those with only one or two copies.
