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A spit test to detect men at increased risk of prostate cancer has started early trials.

The new DNA test looks for high-risk genes that are thought to affect one in every 100 men.

Three hundred men are taking part in the trials, from three London GP surgeries.

Developing better diagnostic tests that could be used as part of a nationwide screening programme is a research priority for prostate cancer.

At present, there is no single, reliable test for prostate cancer. The PSA blood test, biopsies and physical examinations are all used.

But the PSA can give false positives and sometimes misses more aggressive cases.

The new DNA test was created by a group of international scientists based at the Institute of Cancer Research (ICR) in London.

They studied more than 140,000 men and identified 63 new genetic variations that can increase the risk of prostate cancer.

The DNA test combines those variants with more than 100 others previously linked to prostate cancer.

Ros Eeles, professor of oncogenetics at the ICR, said the study was "very significant".

"By looking at the DNA code of tens of thousands of men in more depth than ever before, we have uncovered vital new information about the genetic factors that can predispose someone to prostate cancer, and, crucially, we have shown that information from more than 150 genetic variants can now be combined to provide a readout of a man's inherited risk of prostate cancer."

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Only those men found to be at higher risk of prostate cancer would then be scanned and have a prostate biopsy, so researchers hope it could prevent unnecessary procedures.

"It could have a substantial impact on how we actually manage those at increased risk because if you find the disease earlier it's much easier to treat it and much more easy to cure," says Prof Elees.

The trial will be expanded to 5,000 men next year.

Prof Paul Workman, chief executive of the Institute of Cancer Research, said the study also provided important information about the causes of prostate cancer and the potential role of the immune system "which could ultimately be employed in the design of new treatments".

The study is published in the journal Nature Genetics and was funded by the the National Cancer Institute in the US, with additional support from the European Research Council, Cancer Research UK and Prostate Cancer UK.

Dr Iain Frame, director of research at Prostate Cancer UK said: "This new research could help men to understand their individual genetic risk of prostate cancer, which could prompt them to speak to their GP about the disease.

"Given that one in eight men will be diagnosed with prostate cancer in their lifetime, we urgently need more accurate diagnostic tests which are suitable for use in a nationwide screening programme."

Carl Alexander from Cancer Research UK, said the study was "an exciting example" of how research can find clues in our genes to help us uncover those more likely to develop the disease.

"The next steps should be to understand how this research can be developed into tests which could identify men who might be more likely to develop aggressive cancers, and how this could be rolled out to patients."