PSA test in obese men can produce false results

Nov 20 2007

Scientists in the United States suggest a blood test widely used to screen for prostate cancer can be misleading in the case of men who are obese.

The researchers at Duke Prostate Center, Duke University in North Carolina, say doctors reading the results of a blood test commonly used to screen for prostate cancer can be deceived into thinking obese men are disease-free.

They say the test for a protein called prostate-specific antigen, or PSA, may in fact produce falsely reassuring results because obese people have more blood in their bodies and the concentration of the protein is diluted.

The prostate gland produces PSA and doctors use this to detect the presence of prostate tumors, if levels are higher it can be a sign of cancer; an enlarged prostate can also elevate PSA levels.

The discovery was made when the researchers examined the medical records of nearly 14,000 men who had undergone surgery to treat prostate cancer between 1988 and 2006.

The surgery was carried out at Johns Hopkins Hospital in Maryland, Duke University in North Carolina or five U.S. Veterans Affairs hospitals in California, Georgia and North Carolina.

The researchers found that men with a body mass index, or BMI, indicating obesity, had a higher blood volume and lower PSA concentrations.

The most obese men had PSA concentrations 11 to 21 percent lower than those recorded in men of normal weight.

The researchers say such men could have a total amount of PSA in the blood that might signal prostate cancer, but because they had so much more blood, the PSA concentration was so diluted that the test results seemed to show no cause for alarm.

Dr. Stephen Freedland, one of the research team says the PSA test needs to be adjusted by as much as 15 to 20 percent downwards for obese people or many cancers will be missed.

The prostate is a walnut-sized gland found below the bladder, which produces seminal fluid.

Experts say in view of the rising rates of obesity in the United States and worldwide, the findings are particularly significant.

Dr. Freedland says the findings could affect the way doctors look at other tests for cancer and other diseases that depend on concentrations of disease markers such as PSA in the blood.

He says the findings are very important because of the sheer number of people they affect.

According to the American Cancer Society about 27,000 men will die from prostate cancer in the United States this year and about 219,000 men will be diagnosed with it.

Globally there are 679,000 new cases of prostate cancer diagnosed each year and about 221,000 deaths.

The study was funded by the Department of Veterans Affairs, the Duke Department of Surgery and Division of Urology, the Department of Defense Prostate Cancer Research Program, the American Urological Association Foundation, the Georgia Cancer Coalition, and the National Institutes of Health.

It is published in the Journal of the American Medical Association.