March 10, 2008 -- The PSA test is no longer as useful as it once was as a screening tool for early prostate cancer in men who have normal prostate exams, a new study suggests.
Researchers conclude that changes in biopsy practices over the years have diminished the test's value in predicting prostate cancer.
PSA testing has been widely used for nearly two decades to screen for prostate cancer, and numerous studies have supported the use of the simple blood test as a predictive test for the cancer.
But most of this research on PSA screening was done in the early to mid 1990s, before major changes occurred in prostate biopsy practices.
Before this time, biopsies were rarely performed on men with PSA levels lower than 4, whereas today men with PSAs as low as 2.5 are biopsied. Changes in biopsy practices, including taking more core samples from the prostate, have also led to more cancers being found at earlier stages.
Suspecting that these changes might have changed the predictive value of PSA testing, researcher Douglas S. Scherr, MD, and colleagues from New York's Weill Medical College of Cornell University conducted a study designed to test the theory.
PSA, prostate-specific antigen, is given off by prostate cells. A rise in PSA could mean a prostate tumor is growing.
The problem is that not every man with a high PSA level has prostate cancer. And not every man with prostate cancer has a high PSA level.
"We've been finding at this institution and really nationally and internationally that even men with very low PSAs often have aggressive cancers and men with high PSAs don't have cancer at all," Scherr tells WebMD. "So it is clear that the predictive value of the PSA test is in question."
(If you or a loved one has gone through prostate cancer, what was your experience when going through screening? Share your thoughts with others on WebMD's Prostate Cancer: Support Group message board.)
PSA Over Time
Scherr and colleagues reviewed 1,607 patients who had prostate biopsies done at Cornell's New York Presbyterian Hospital between 1993 and 2005, dividing the patients into three groups based on when biopsies were performed (1993-1997, 1998-2001, and 2002-2005).
They found that the number of biopsies performed, the percentage of positive biopsies showing cancer, and patient ages did not change significantly over time.
But there was a significant decrease in the average PSA level among biopsied patients and an increase in the number of core samples taken during biopsies.
Over the entire study period, the positive biopsy rate in men with PSA levels between 2 and 4 and normal prostate exams was not significantly different from that of men with higher PSA levels and normal prostate exams.
The study is published in the March 15 issue of the American Cancer Society (ACS) journal Cancer.
Scherr tells WebMD that whether the PSA was as low as 2.5 or as high as 10, the likelihood of finding cancer was about the same in the biopsied patients.
"So the utility of using PSA to weed out who does and does not need a biopsy has really been lost given the fact that the prostate cancers we are finding tend to be smaller than they were," he says.
Should We Biopsy Everyone?
The Cornell researchers conclude that better molecular markers are needed to screen for prostate cancer.
"Given the fact that PSA has far less predictive value than it had in the early '90s, the next question is, can we replace it?" Scherr says.
One potential approach, he adds, is to perform prostate biopsies on all men when they reach the age of 50, in the same way that colonoscopy is recommended to look for colon cancer.
"I'm not recommending this, but we are beginning to look at this question," he says. "If the biopsy is the ultimate arbiter, maybe we should move in the direction where we biopsy everyone."
ACS Director of Screening Robert A. Smith, PhD, says more biopsies would likely lead to the detection of more very early-stage prostate cancers.
"PSA is an imperfect test for the early detection of prostate cancer, but it happens to be the best one we have at the moment," he says. "Because of the uncertainties, leading cancer organizations, including ACS, recommend that men discuss the pros and cons of this test with their doctors."
PSA researcher Andrew Vickers, PhD, tells WebMD that PSA testing is useful in predicting, decades before the fact, which men will develop prostate cancer.
The Memorial Sloan-Kettering Cancer Center statistician co-authored a study published last month suggesting that a single PSA test taken at age 50 or before is a very strong predictor of who will have prostate cancer up to 25 years later.
Vickers says PSA may be a useful test in young men, to identify those who might benefit from aggressive screening.
"In this study, 50% of the advanced cancers were diagnosed in men in the top 10% of PSA levels in that early screening, and two-thirds of the advanced cancers were seen in the 20% of men with the highest PSAs," he tells WebMD. "This shows that a single PSA prior to age 50 can predict extremely well who will eventually have advanced prostate cancer."