Obesity can skew key prostate cancer test results

Obesity can skew key prostate cancer test results
WASHINGTON (Reuters) - Doctors reading the results of ablood test widely used to screen for prostate cancer can befooled into thinking obese men are disease-free, researcherssaid on Tuesday.
The test may yield falsely reassuring results because obesepeople have more blood in their bodies due to their girth, thusdiluting the concentration of the protein doctors use to detectthe presence of prostate tumors, the researchers said.
The prostate gland produces a protein calledprostate-specific antigen, or PSA. Only prostate cells produceit and if levels are higher it suggests the cells are growing-- which can be a sign of cancer although an enlarged prostatecan also send PSA levels up.
The researchers examined medical records for nearly 14,000men who had undergone surgery to treat prostate cancer between1988 and 2006 at Johns Hopkins Hospital in Maryland, DukeUniversity in North Carolina or five U.S. Veterans Affairshospitals in California, Georgia and North Carolina.
Men with a body mass index, or BMI, indicating obesity hada higher blood volume and lower PSA concentrations. The mostobese men had PSA concentrations 11 to 21 percent lower thanthose recorded in men of normal weight, the researchersreported in the Journal of the American Medical Association.
These men could have a total amount of PSA in the bloodthat might signal prostate cancer, but because they had so muchmore blood, the PSA concentration was so diluted that the testresults seemed to show no cause for alarm, they added.
Thus, PSA concentrations that might be no worry for a thinman might suggest cancer for an obese one. "It's not that PSAis a bad test in obese men. Rather, we just need to learn howto use it better," Duke urologist Dr. Stephen Freedland, one ofthe researchers, said in a telephone interview.
"So whatever (PSA level) you consider abnormal, you justhave got to adjust it by about 15 to 20 percent downwards forobese people," Freedland added, or risk missing many cancers.
The prostate is a walnut-sized gland that produces seminalfluid. It is found below the bladder.
Dr. Carmen Rodriguez, an American Cancer Societyepidemiologist who participated in the study, said the findingswere particularly important considering the rising rates ofobesity in the United States and worldwide.
Rodriguez said doctors had known obese men were at higherrisk of developing more aggressive prostate cancer. She saidthis study indicates one of the reasons may be that some obesemen could have had false negative results in PSA tests, withtheir cancer then detected much later after it had grown moreadvanced and more dangerous.
Freedland said the findings could affect the way doctorslook at other tests for cancer and other diseases that dependon concentrations of disease markers like PSA in the blood.
He said it might be helpful to consider the total amount ofa disease marker in the body rather than its concentration in acertain volume of blood, in order to account for the dilutionthat can occur in the obese.
Worldwide, prostate cancer is estimated to kill about221,000 people annually, with 679,000 new cases diagnosed.
The American Cancer Society estimates that about 27,000 menwill die from prostate cancer in the United States this yearand about 219,000 men will be diagnosed with it.
(Editing by Maggie Fox)