Complicating Disease Detection

April 1, 2008

Doctors may be missing prostate cancers in obese men because the telltale blood marker used to detect the disease can be falsely interpreted as low in this population, according to a new study led by Duke Prostate Center researchers.

"Obese men have more blood circulating throughout their bodies than normal-weight men, and as a result, the concentration of prostate-specific antigen, or PSA, in the blood—the gold standard for detecting prostate cancer—can become diluted," says Stephen Freedland, assistant professor of urology and senior researcher on a study published in the Journal of the American Medical Association.

"We've known for a while that obese men tend to have lower PSA scores than normal-weight men, but our study really proposes a reason why this happens and points to the need for an adjustment in the way we interpret PSA scores that will take body weight into account."

Researchers compared the medical records of almost 14,000 patients who had undergone radical prostatectomy surgery for the treatment of prostate cancer between 1988 and 2006 at Johns Hopkins, Duke, and five Veterans Affairs hospitals. They analyzed the relationship between body mass index—which is a measure of obesity—and PSA concentration levels, while also examining the blood volume in the patients' bodies and the total amount of PSA protein found in the blood, known as PSA mass.

They found a direct correlation between higher body mass index, higher blood volume, and lower PSA concentration. In this study, PSA mass across all groups was comparable, despite differences in body weight, leading the researchers to believe that the larger blood volume is responsible for lowering the concentration of PSA, which is what doctors typically measure when looking for prostate cancer, Freedland says.

The study illustrates a potentially serious consequence of the obesity epidemic, says Carmen Rodriguez, an American Cancer Society epidemiologist and co-author of the study, adding that other studies have linked obesity to more aggressive prostate cancers. If their prostate cancers are being detected later because of the dilution of PSA, this may help, in part, to explain why obese men tend to have more aggressive cancers, Freedland says.