CHICAGO (Reuters) - A highly sensitive blood test may be able to predict whether prostate cancer is cured or is likely to come back, giving doctors an early sign of whether treatments are working, US researchers said on Wednesday.

They said the nanotechnology-based blood test is far more sensitive than currently available commercial tests for prostate-specific antigen, or PSA, a protein produced by cells in the prostate gland.

"Our assay can detect PSA in blood samples 300 times better than the current standard PSA tests," Dr. Shad Thaxton of Northwestern University's Feinberg School of Medicine in Chicago said in a telephone interview.

The test is not intended for routine prostate cancer screening, but is meant to look for signs that prostate cancer has spread to other organs in patients who have had their prostate gland removed.

"Through the technology, it appears we will be better at determining which patients are cured and which patients are destined for prostate cancer recurrence," said Thaxton, whose study was presented at the American Urological Association meeting in San Francisco.

"It may allow physicians to act at the earliest and most sensitive time, which we know will provide the patient with the best chance of long-term survival," he said.

The new test takes advantage of some special properties of gold nanoparticles — which are 1,000 to 100,000 times smaller than the width of a human hair.

"Gold nanoparticles have the ability to act as catalysts, and they interact very intensely in light," Thaxton said.

To make a blood test, the team added antibodies onto the surface of the gold particles, which seek out and stick to tiny bits of PSA in the blood.

Men who have had the prostate removed because of cancer should have only trace amounts of PSA in their blood. But if a prostate tumor has spread to other organs, it may be producing PSA. That is what the new test is looking for, Thaxton said.

The team studied the test using frozen blood taken from men after prostate surgery whose blood had tested clean for PSA using standard tests.

The new test showed that low or steady PSA levels meant that the prostate cancer was gone 10 years later. But in men whose PSA level was higher than expected, prostate cancer was more likely to come back.

Prostate cancer is the second most common cancer in men worldwide after lung cancer, killing 254,000 men a year.