A relatively inexpensive blood test appears to be the most accurate means of detecting prostate cancer at an early and curable stage, researchers said.

A study involving nearly 2,000 men found measuring levels in the blood of a protein called prostate-specific antigen - PSA - is the best single screening tool for detecting the second leading cancer killer of American men, Dr. William Catalona of Washington University in St. Louis said Wednesday.It is estimated that more than 100,000 new cases of prostate cancer occur in the United States annually, with more than 30,000 deaths. Catalona said the cure rate is "excellent" when the disease is diagnosed early, but much poorer when the cancer has spread beyond the prostate.

The prostate is a walnut-sized gland at the base of the pelvic cavity that produces fluid that helps transport sperm. For unknown reasons, the gland frequently enlarges in older males and cancer develops in about one of 11 men.

Currently, the standard method of detecting prostate cancer is a "digital rectal examination" in which doctors rely on feeling enlargements that could be cancerous tumors.

However, in seven of 10 men, rectal examinations catch the cancer only when it has spread beyond the prostate, Catalona said.

Among men known to have prostate cancer, the PSA test detected the disease when rectal examinations alone would have missed it more than 30 percent of the time, Catalona and colleagues reported in The New England Journal of Medicine.

The researchers said the results indicate that combining a PSA test with an annual rectal examination in men over 50 is the best means of detecting prostate cancer at a stage in which it can be effectively treated with surgery and radiation.

Although the PSA test has been used widely to monitor the progress of prostate cancer after it has been diagnosed, Catalona said this is the first large study in which the test was used to detect disease.

The study found that when blood levels of PSA, which is produced only in the prostate, exceed 4 nanograms per milliliter men are likely to have some type of prostate problem.

The study compared 1,653 men ages 50 and older with 300 men in the same age bracket whose rectal examinations had indicated a likelihood of cancerous or non-cancerous prostate disease.

Altogether about 10 percent of the men had PSA measurments above normal. In both groups, about one-fourth of those who had PSA levels considered mildly elevated were found to have prostate cancer, Catalona said.