May 20, 1998
A new prostate specific antigen (PSA) test that measures the percentage of "free" PSA in the blood not bound to other proteins could spare up to 200,000 men a year in the United States the pain, anxiety and inconvenience of a surgical biopsy to detect cancer.
Results of multicenter trials of the test, funded by its makers, Hybritech Inc., of San Diego, Calif., (a subsidiary of Beckman-Coulter Inc.), appear in the May 20 issue of the Journal of the American Medical Association.
According to Alan Partin, M.D., associate professor of urology at Johns Hopkins, one of the several centers involved in the trials, the new test should only apply to men whose prostates feel normal on digital rectal exams, but whose standard PSA levels are elevated and between four and 10. "These are the men in the diagnostic gray zone. They're at risk for prostate cancer on the basis of the PSA levels, but experience shows that 75 percent of the time, their biopsies will be negative for cancer," he says. The researchers were led by William J. Catalona, M.D., of Washington University School of Medicine (St. Louis, Mo.).
The test measures a form of PSA, called free PSA, that is lower in men with cancer than in those without cancer or with benign prostate enlargement, says Partin. "Now we can figure with more accuracy who not to biopsy," he says. In the past, he adds, using the percentage of free PSA (%FPSA) to determine risk was clouded by such factors as prostate size and the total amount of PSA.
"The study demonstrated that men in the diagnostic gray zone should probably be biopsied only if their %FPSA is at or below 25 percent. At that level, we can detect 95 percent of cancers while excusing men with benign diseases from the need for a biopsy," Partin says.
Prostate cancer screening tests, Partin says, must be sensitive enough to find as many potentially curable cancers as possible, even if they fail to detect microscopic, inactive cancers that pose no threat. "By using this test to zero in on men in a certain group most likely to have active cancer, there is the potential to cut the cost of prostate screening dramatically," says Partin. "We also spare those men the possible complications of biopsies, such as infection, bleeding and fever."
The test also can help determine the need for repeat biopsies. For example, if a man's first biopsy is negative, but his father and brothers have prostate cancer and his %FPSA value is 25 or less, he still might be advised to undergo a second biopsy. "About 20 percent of cancers are missed on the first biopsy, so having a %FPSA measurement test will help us decide if a second biopsy is necessary," he said.
In the study, the test for %FPSA was tried on blood samples from 773 men, ages 50 to 75, who had normal results on digital rectal exam and had received no treatment for prostate disease. Because all had a previous record of PSA levels between four and 10, all had been biopsied. A total of 379 were found by the test to have prostate cancer, while 394 men did not.
Other authors of the study include Patrick C. Walsh and Eric N. P. Subong (Johns Hopkins); Kevin M. Slawin and Peter T. Scardino (Baylor College of Medicine, Houston, Texas); Michael K. Brawer and Paul H. Lange (University of Seattle, Wash.); Robert C. Flanigan (Loyola University Medical Center, Maywood, Ill.); Anup Patel and Jean B. DeKernion (University of California, Los Angeles); Jerome P. Richie (Brigham and Women's Hospital, Boston); and Robert E. Parson, Gail H. Gasior, Kathleen G. Loveland and Paula C. Southwick (Hybritech).