BLOOD TEST FOR PROSTATE CANCER MORE EFFECTIVE THAN A PHYSICAL

Researchers say a blood test is twice as good as a physical exam at detecting prostate cancer. However, another blood test, for colon cancer, was pronounced virtually worthless.

Both studies were published Wednesday in The Journal of the American Medical Association.One found that a test that measures elevated levels of a certain protein in the blood is twice as effective as the usual rectal exam at detecting prostate cancer in its early stages. Prostate cancer will kill 35,000 American men this year.

More than 10,000 men participated in the study of the test, which measures levels of prostate-specific antigen, or PSA, a protein that seeps out of the walnut-sized gland if a tumor is present or the gland is enlarged.

The study was led by Dr. William J. Catalona of the Washington University School of Medicine in St. Louis and was paid for the company that makes the PSA test and by the government.

The other study criticized a blood test commonly used to detect new malignancies after surgery for colon cancer - the second leading U.S. cancer killer, behind lung cancer. It will kill about 57,000 Americans this year.

About 500,000 Americans now get the colon cancer blood test at least once and possibly many times after colon-cancer surgery, the researchers said.

The $55 test, called CEA for "carcinoembryonic antigen," measures blood levels of a carbohydrate-protein molecule that colon cancers may produce in large quantities.

The test misses many recurrences of cancer. And in other patients, it often suggests the presence of cancer when none exists, the researchers found.

When the test result is right, it often is too late. When it is wrong, far more expensive tests and even surgery are needed to rule out cancer, they said.

The researchers, led by Dr. Charles G. Moertel of the Mayo Clinic, studied 1,216 colon-cancer patients treated after surgery in a number of states.

The prostate-cancer study, conducted from 1989 to 1992 in St. Louis, compared 10,251 healthy men over age 50 who underwent PSA screening to 266 comparable men who did not but had been referred because of abnormal rectal exams.